President Obama's brave and surprising inaugural address was the 56th of its kind since George Washington delivered the first one in New York on 20 April 1789. Over the last two centuries, these speeches have become as thickly encrusted with conventions as the limerick, the sonnet or the Times crossword: they are a bizarre literary form, unique to the United States, with a tiny handful of acknowledged classics (Lincoln's two inaugurals, Franklin Roosevelt's first, and John F Kennedy's solitaire) that stand proud of the generally depressing mass of mediocre and bombastic writing, most of which now reads like cold porridge.

Since Andrew Jackson's time, inaugurals have been designed for outdoor delivery to an often wet and shivering crowd in Washington's capricious weather. Even before Inauguration Day was set back on the calendar from 4 March to 20 January in 1937, the ceremony seems to have been a magnet for snow, rain and frigid northeasterly gales. Obama was lucky: the sun shone, though the temperature in the National Mall was a forbidding 28F.

At William Henry Harrison's March inauguration in 1841, he stepped to the podium without hat or overcoat, under a louring sky, his trouserlegs flapping in the bitter wind, and delivered the longest address on record. At nearly 8,500 words, it took Harrison 100 minutes to read aloud to a fast-diminishing audience. When he died the following month, of pneumonia and pleurisy, it was widely believed that his rash feat of oratory must have been the cause. In fact, he first showed symptoms of a cold nearly two weeks after the event, but the idea that a presidential inaugural can be fatal has gained such a hold on the American imagination that the legend of Harrison's death-by-speechmaking lives on, despite the medical evidence against it.

Nineteenth-century presidents could count on crowds of up to 10,000 people milling around the east portico of the Capitol building; Franklin Roosevelt spoke to 100,000 in 1933, and more than a million showed up for Lyndon Johnson's inauguration in 1965. At least half a million came when Ronald Reagan changed the venue to the west portico in 1981, enabling the president to speak to the nation looking symbolically westward, down the Mall, past the Washington and Lincoln monuments, and through the earth's curvature over three time zones to the Pacific coast. Crowd estimates are always unreliable, but Obama is said to have drawn 1.8 million people on Tuesday, which, if true, is an all-time record.

The growing crowds and open-air character of the occasion help to account for the ever-increasing grandiosity of language in inaugural addresses. The words of most inaugurals would sound insane if spoken quietly, indoors; and they sometimes sound that way when carried on the wind via loudspeakers and vast JumboTron TV screens. These quadrennial speeches are as close as America comes to the rhetoric heard more than 70 years ago in Red Square, the Piazza Venezia in Rome and the Reichsparteitagsgelände in Nuremberg. Or, to put it in the more kindly words of Peggy Noonan, the columnist and former Reagan speechwriter, they are "a golden trumpet that a president gets to blow at most twice in his life".

Four years ago, when George W Bush lifted that trumpet to his lips, he used it to declare a global war on tyranny. "America, in this young century, proclaims liberty throughout all the world, and to all the inhabitants thereof. Renewed in our strength - tested, but not weary - we are ready for the greatest achievements in the history of freedom." Try saying that across the kitchen table, paying particular attention to your pronunciation of "thereof", then wait for the arrival of the men in white coats.

Rhetorically speaking, much - too much - is expected from a speech delivered from behind shields of bulletproof glass and introduced by the band of the US Marines playing "Hail to the Chief", the music punctuated by gunfire from saluting cannons. Because the president's language has to struggle to maintain the ceremonial pomp of the occasion, lofty similes and metaphors are the order of the day, and classical tropes, with names like Aegean islands, from Anaphora to Zeugma, are retrieved from storage to give the oration an air of immemorial antiquity.

Inaugurals conventionally start with a history lesson and finish with a prayer. In the first paragraphs, the newly sworn-in president thanks his predecessor for his service to the nation, applauds the miracle of a peaceful transition (as if all other countries went in for putsches and coups), and reminds America of its unique place as the cradle of modern democracy, summoning the ghosts of his illustrious predecessors, Washington, John Adams, Jefferson, Madison and Lincoln, to crowd around him in a Mount-Rushmore-like tableau. In the last paragraph, it's customary to call on God to bless the nation and the great enterprise of the incoming administration, addressing him by such honorifics as "the benign Parent of the Human Race" (Washington), "that infinite Power" (Jefferson), "the kind Providence" (Pierce), "the giver of Good" (Theodore Roosevelt), "the Author of Liberty" (George W Bush).

Sandwiched between these more or less canonical beginnings and endings is the less predictable meat of the piece. Most often it's a mission statement, couched in terms of uplifting generality and promising great good for all in the sunlit years ahead. Clinton's second inaugural in 1997 was a dismal classic in this regard. Although he did try to address the issue of race, the majority of the speech was boilerplate: "We need a new sense of responsibility for a new century . . . Our greatest responsibility is to embrace a new spirit of community for a new century . . . The challenge of our past remains the challenge of our future . . . With a new era of government, a new sense of responsibility, a new spirit of community, we will sustain America's journey." Words that were already dead on the page died a second death as Clinton gave them voice. Coming from someone with a reputation for effortless, improvised rhetoric, the speech was shockingly empty: the liberal historian William Leuchtenburg called it "the most banal address by an American president I have ever heard".

It generally takes an imminent catastrophe to persuade the new president to talk in detail about current events. Lincoln, facing the coming civil war in 1861, mounted an impassioned and closely argued defence of the "perpetuity" of the union and the constitution against the Southern secessionists, in a speech that's still ablaze with life today. In 1933, Franklin Roosevelt crisply laid out the economic theory and practice of the New Deal in less than 2,000 words, weaving farm prices, factories, mortgages, foreclosures, jobs and public works into an instantly comprehensible plan of action, which took immediate effect by lifting the spirits of a profoundly depressed nation. But these are famous exceptions to the tradition of the inaugural as an exercise in ghostwritten magniloquence.

Until very recently, the pretence was kept up that these speeches were the original work of the president-elect, scribbling alone up in his den or - in JFK's case - the locked cabin of a private yacht, surrounded by blunted pencils and scumbled pages from a yellow legal pad. But this has always been a polite fiction.

When Lincoln showed the first draft of his 1861 inaugural to William Seward, his chief rival for the Republican nomination and his designated secretary of state (yes, history does sometimes repeat itself), Seward returned it with a sheaf of corrections to almost every sentence ("Strike out the whole paragraph", "For 'treasonable' write 'revolutionary'") and a drastically changed ending. Lincoln adopted almost all of Seward's suggestions, including the most important one, where Seward insisted that the speech end not on a challenge to the South (Lincoln had written "Shall it be peace, or a sword?") but on an appeal to what he called "the mystic chords" of shared historical memory. Seward's mystic chords went in, as did his hearts and hearths, his patriot graves and battlefields, his bonds of affection, his angels and ancient music, but as Lincoln rewrote Seward he sharpened every idea and phrase, giving the new ending a poignancy and intimacy of tone that hugely improved on Seward's original.

Lincoln made Seward's words his own, but most presidents, although they tinker with their speeches, leave the business of writing to their ghosts, and sometimes, on Inauguration Day, appear baffled by the strange-tasting language they find in their own mouths. In January 1965, with Mariner 4 en route to Mars, Lyndon Johnson haltingly read aloud this lyrical paean, probably written by Richard Goodwin, LBJ's leading spook:

"Think of our world as it looks from the rocket that is heading toward Mars. It is like a child's globe, hanging in space, the continents stuck to its side like coloured maps. We are all fellow passengers on a dot of earth."

His disbelief was audible. So was George HW Bush's, in 1989, when he found himself telling the children of America that "Democracy belongs to us all, and freedom is like a beautiful kite that can go higher and higher with the breeze."

The two best inaugurals of modern times were written by ghosts. Raymond Moley, a former professor of politics at Columbia, drafted Roosevelt's 1933 address, and its best-known phrase, "the only thing we have to fear is fear itself", was slipped in at the last moment by another aide, Louis Howe. As for Kennedy's 1961 rhetorical triumph, his chief speechwriter, Ted Sorenson, was recently questioned by Deborah Solomon of the New York Times, who asked him point-blank if he was the true author of "Ask not what your country can do for you . . ." His succinct reply was "Ask not".

The trouble with ghostwriting is that it raises the issue of whether the president is in a state of diminished responsibility for what he says. Does he actually grasp the implications of the words he speaks? A case in point is FDR's attack on bankers in his first inaugural:

"Practices of the unscrupulous money changers stand indicted in the court of public opinion, rejected by the hearts and minds of men . . . They know only the rules of a generation of self-seekers. They have no vision, and when there is no vision the people perish . . . The money changers have fled from their high seats in the temple of our civilisation. We may now restore that temple to the ancient truths."

The embedded quotation from the Book of Proverbs ("They have no vision") only helps to underline the antisemitism of the passage. But Roosevelt himself was not an antisemite; his closest friends, his cabinet and his Supreme Court appointments included many Jews, such as Felix Frankfurter, Henry Morgenthau, Abe Fortas and Louis Brandeis.

Yet calling bankers "the money changers", along with talk of driving them from the temple, was a favourite trope of Father Coughlin, the explicitly antisemitic demagogue and "America's radio priest". Coughlin, who supported Roosevelt in the 1932 election against Hoover, broke with him shortly afterwards, deriding the New Deal as the "Jew Deal". The lightly encoded message about an international conspiracy of Jewish bankers, written into Roosevelt's speech by Moley, ought to have been recognised by the president for what it was and immediately struck out. But it remained.

It's a puzzle. Perhaps the period explains it; a time when genteel antisemitism was so routine that it passed unnoticed. Perhaps - though this seems most unlikely - Roosevelt was in secret agreement with Coughlin's paranoid tirades. Perhaps he just warmed to the biblical roll and grandeur of the words when Moley showed him the script: vision, money changers, temple, ancient truths - powerful inaugural stuff. Whatever its explanation, this curious passage illustrates the danger of a president becoming the unwitting puppet of his ghosts, as I believe George W Bush did in 2005 with his neoconservative, Project for the New American Century, Weekly Standard-style inaugural and its - mercifully empty - promise to bring about democracy around the globe by force of American arms.

No recent inaugural has been as keenly anticipated as Obama's. On the strength of Dreams From My Father, he's not only the best writer to occupy the White House since Lincoln (not a title for which there's stiff competition), he's also the most rousing American political orator of his generation. The big question was: would he write it himself? To which the disappointing answer was a qualified no.

It was probably because Obama first made his name as an eloquent writer that in December the Washington Post broke the convention of tactful silence on the issue of ghostwriting, and published a long and revealing article about Obama's relationship with Jon Favreau, his chief speechwriter. Favreau is now 27, and has been ghosting for Obama since 2004. He was found by the Post reporter in a local Starbucks, working at a laptop on a document headed "ROUGH DRAFT OF INAUGURAL", having missed his original deadline of Thanksgiving.

Unlike Sorenson, Favreau (otherwise "Favs") wasn't coy about his job as presidential dramaturge, chatting freely about how he'd been listening to recordings of previous inaugurals and had paid a visit to Peggy Noonan to get the inside dope on how to write one. (Bad news, this: the modern inaugural address is a form that cries out to be broken, not copied.) The reporter, Eli Saslow, described Obama's and Favreau's usual procedure as they work up a speech:

"Before most speeches, Obama meets with Favreau for an hour to explain what he wants to say. Favreau types notes on his laptop and takes a crack at the first draft. Obama edits and rewrites portions himself - he is the better writer, Favreau insists - and they usually work through final revisions together. If Favreau looks stressed, Obama sometimes reassures him: 'Don't worry. I'm a writer, too, and I know that sometimes the muse hits you and sometimes it doesn't. We'll figure it out together.'"

Favreau is said to travel everywhere with a copy of Dreams From My Father, written before Obama entered politics, using it as the key to his master's authentic voice. He has internalised Obama's speech patterns along with his biography, and can now impersonate Obama on the page, speaking in the first person singular, with uncanny plausibility. Favreau says that when he leaves the White House he'd like to write "a screenplay or maybe a fiction book based loosely on what all this was like". But - whether or not he knows it - he is writing fiction now, losing himself inside a character remote from his own, as playwrights do. It's a bit like Alan Bennett writing Miss Fozzard and the other monologuists in his Talking Heads series: Favreau is white, single, 20 years younger than Obama, a buzzcut Generation Y-er, whose chief amusement, when not channelling Obama on his laptop, is all-night videogaming. (He caused a momentary scandal when the press found on his Facebook page a silly photo of him groping the right breast of a cardboard cutout of Hillary Clinton, while a friend jammed a bottle of beer against her face.)

The gulf in age and temperament between Obama and Favreau is probably a liberation for the speechwriter. To inhabit the skull of someone different from oneself is one of the great pleasures of writing fiction, and the character of Obama (should that be "Obama"?) must be fun to work with: his natural gravity, his unflappable cool, his perpetually wrinkled brow, his sudden flashes of self-depreciating humour ("a mutt like me . . ."), his ability to switch styles, from law professor to black preacher and back again, his rich and flexible actor's baritone. No president since Kennedy has given his ghost so much to exploit, so many opportunities for elaborate verbal invention.

I suppose it's naive to be disconcerted by the fact that Obama employs ghosts (Favreau heads a team of them), but his best speeches have been so personal, so drenched in the past he described in Dreams From My Father, that one can't help feeling a little let down to learn that, for instance, his masterful and exhilarating speech on race, delivered last March in Philadelphia, was a joint Obama/Favreau production. From the Washington Post:

"One Saturday night in March, Obama called Favreau and said he wanted to immediately deliver a speech about race. He dictated his unscripted thoughts to Favreau over the phone for 30 minutes - "It would have been a great speech right then," Favreau said - and then asked him to clean it up and write a draft. Favreau put it together, and Obama spent two nights retooling before delivering the address in Philadelphia the following Tuesday."

I think it's fair to assume that Favreau was here minimising his own role in the composition of the speech. But, however one reads the account, what is one to make of this much-quoted passage:

"I can no more disown [the Rev Jeremiah Wright] than I can disown the black community. I can no more disown him than I can my white grandmother - a woman who helped raise me, a woman who sacrificed again and again for me, a woman who loves me as much as she loves anything in this world, but a woman who once confessed her fear of black men who passed by her on the street, and who on more than one occasion has uttered racial or ethnic stereotypes that made me cringe. These people are a part of me. And they are a part of America, this country that I love ..."

As soon as one becomes aware of the presence of Favreau's fingers on the keyboard, the can of Red Bull, his preferred energy drink, beside him on the table, and the whiteness of his skin, the questions multiply: does this sound like Obama talking over the phone to Favreau? Obama recalling his Hawaiian childhood in his own words? Or just Favreau recalling Obama's 1995 memoir (which he must now know by heart)? As Bill Clinton almost said, it all depends on what the meaning of I is.

The aim of Obama's inaugural, he told George Stephanopoulos on ABC's This Week, was "to try to capture as best I can the moment we are in". It would have been a more obviously arresting speech if he'd tried to capture the moment in the language of the present century instead of using the faux-antique dialect of past inaugurals. So many phrases had the dull patina of silver that has jingled in dead presidents' pockets. The few mint coins in his oration stood out by their brightness, like "our patchwork heritage", followed by the addition of "non-believers" to "We are a nation of Christians and Muslims, Jews and Hindus" - the first time that atheism has been included under the rubric of religious tolerance and freedom. And there was the pretty rhetorical flourish of "The nation cannot prosper long when it only favours the prosperous".

The surface tone of the address was set by somewhat moth-eaten metaphors ("rising tides of prosperity and the still waters of peace . . . amidst gathering clouds and raging storms") and a curious solecism in its sixth sentence: "At these moments, America has carried on not simply because of the skill or vision of those in high office, but because we, the people, have remained faithful to the ideals of our forbearers, and true to our founding documents." Forbearers? In neither American nor British English is a forbearer a forebear, or ancestor; it's a person who shows forbearance - endurance under provocation. But someone (Favreau? Obama?) must have thought that the extra syllable in "forbearer" gave the word the sort of solemn weightiness suitable to the liturgical grandeur of an inaugural address, and tipped it into the speech without regard for its actual meaning. Its presence in the speech as delivered and distributed to the press (the White House has since corrected its official text) reveals just how anxious the authors were to uphold the stilted linguistic conventions of the form - and for good reason.

Under the guise of noble platitude Obama was able to get away with murder, cloaking in familiar and emollient language an address that otherwise defied convention. There was a hint of this in his ritual bow to the outgoing president, in which he spent five words acknowledging Bush's service to his country and 10 in thanking him for his departure from office. In no inaugural has a president so completely repudiated the policies of his predecessor as Obama did on Tuesday. Look back at the "forbearers" sentence, and see the sting in its tail: "true to our founding documents". Most of the crowd of more than a million who packed the Mall, a few of whom loudly booed Dick Cheney when he was wheeled on to the stage, believed that the Bush administration had done its best to shred the constitution. The distinction between "we the people" (who are loyal to the founding documents) and "those in high office" (who stand accused of abusing them) hung ambiguously in the air. If you wanted to hear it, it was there; if you didn't, it wasn't.

This veiled quality suffused the entire address, whose central motif was stated early on: "The time has come to put away childish things . . . to reaffirm our enduring spirit; to choose our better history." The distinct echo of Lincoln's "the better angels of our nature" helped to soften the implication that the last eight years belong to our worse history, under a president famous for his childish pursuits (as one of Bush's own advisers once asked, "What kind of male obsesses over his bike-riding time, other than Lance Armstrong or a 12-year-old boy?").

Yet there was no triumphalism in this; there was, rather, a note of sombre regret. It was necessary for Obama to announce to both the United States and the rest of the world (and his inaugural was directed, unusually, at least as much to the foreign as to the domestic audience) that on Tuesday the Bush era had ended and that America, after a long, unhappy detour in the wilderness, was returning to its better history. Since inaugural addresses are by tradition high-toned, bipartisan affairs, this was an immensely difficult feat to bring off with grace. What needed to be said had to be phrased in language as well-worn and conventional as possible, to give the illusion of smooth continuity between Obama's speech and those of past presidents.

The driving theme of the address made its appearance at artfully calculated intervals, with Obama touching on it, departing from it, returning to it, burying it for a while and digging it up again in a way that made some critics call the speech diffuse. But it was not diffuse. It was quietly, courteously insistent on its purpose.

"On this day, we come to proclaim an end to . . . the worn-out dogmas that for far too long have strangled our politics": an end, then, to the liberal imperialism of the neoconservatives and the "Bush doctrine" of preemptive invasion. "Starting today, we must pick ourselves up, dust ourselves off and begin again the work of remaking America": so, under the 43rd president, we have been floored and supine. The most damning censure of the Bush administration arrived exactly midway through the speech, at the nine-and-a-half minute mark:

"As for our common defence, we reject as false the choice between our safety and our ideals. Our founding fathers, faced with perils that we can scarcely imagine, drafted a charter to assure the rule of law and the rights of man, a charter expanded by the blood of generations. Those ideals still light the world, and we will not give them up for expedience's sake. And so to all the other peoples and governments who are watching today, from the grandest capitals to the small village where my father was born: know that America is a friend of each nation and every man, woman and child who seeks a future of peace and dignity, and we are ready to lead once more."

That is as near as George W Bush has come to being impeached. It covers the legal black hole of Guantánamo Bay and its kangaroo courts, the overreaching powers of the Patriot Act, torture, warrantless wiretapping and all the other infractions of the civil liberties of Americans and foreigners alike that occurred under the outgoing administration. "We are ready to lead once more" is startlingly candid in its admission that, under Bush, the United States did not lead the world but attempted to bomb and bully it into submission.

The reference to "the rights of man" was salient. The title of Thomas Paine's giant pamphlet prepared the way for Paine's incognito appearance at the end of the speech, when Obama talked of Christmas night in 1776, when George Washington led his ragtag army across the ice-choked Delaware river to confront the British and Hessians who were encamped at Trenton, New Jersey. Obama spoke of "the timeless words" that "the father of our nation" ordered to be read to the American people: "Let it be told to the future world, that in the depth of winter, when nothing but hope and virtue could survive, that the city and the country, alarmed at one common danger, came forth to meet [it]." The oddly bracketed "it" replaced the original end of the sentence, which was "came forth to meet and to repulse it."

Every commentator I heard - including, surprisingly, the historian Doris Kearns Goodwin - assumed that the quotation came from Washington himself, but it is from Paine's The Crisis. "With hope and virtue, let us brave once more the icy currents," Obama said, and, again, the commentators assumed, mistakenly I think, that he was speaking only of the recession as it deepens, with increasing speed, into depression. But Paine's authorship of those words suggests otherwise. The "common danger", requiring "hope" and, more pointedly, "virtue" in order to "meet [and to repulse] it", is surely as much the spectre of a dictatorial administration, emboldened by Dick Cheney's theory of the "unitary executive", and its dangerous freedom to abuse the rights of man, as it is the present economic crisis. No wonder "and to repulse" was left out: Obama, a cautious politician and sensitive to the nuances of words, stopped short of calling the Bush administration repellent.

His image of leaving the blood-stained snow behind to cross the freezing river - famous from schoolroom reproductions of Emanuel Leutze's 1851 painting George Washington Crossing the Delaware - and his marriage of December 1776 to "this winter of our hardship" were his most daring attempts at inaugural loftiness. But there was more to them than a stirring call to arms to fight recession; he was placing between his incoming administration and that of the outgoing president a broad river packed with growling chunks of ice - a river just crossed, at great hazard to the survival of America's "founding documents".

I've read - or at least skimmed - every inaugural address since George Washington's, and none comes close to so categorically rejecting the political philosophy and legislative record of the previous occupant of the White House. Obama did it by stealth - so much stealth that most of the red meat of the speech has so far passed largely unnoticed. The most astonishing visual moment of the inauguration came after the speech, and Elizabeth Alexander's dud poem ("On the brink, on the brim, on the cusp . . .", mistaking her thesaurus for her muse), and the Rev Joseph Lowery's magnificent, scene-stealing benediction, when Obama and his wife Michelle walked George W and Laura Bush to the US Marine helicopter parked beside the Capitol's west portico. The two couples joked, then hugged, before the Bushes climbed aboard, on their way to Midland, Texas. It was like seeing Mark Antony and Brutus locked in a warm embrace after "Friends, Romans, countrymen . . ."

No one will say of Obama's inaugural, as the Atlanta Constitution said the next day of Roosevelt's 1933 speech, "The address takes its place among the greatest of historic state papers of the nation, ranking with Lincoln's address at Gettysburg". Even on Inauguration Day itself, when the press usually takes a rosy view of whatever is said by the new president, journalists grumbled that Obama's oration, though predictably well delivered, was short of specifics, fire and memorability. They searched the text for phrases to stand beside "the only thing we have to fear . . ." or "Ask not what your country . . .", and came away empty-handed. Conservative journalists noted that Obama had taken some "digs" at Bush, but failed to read the truly damaging subtext.

Among the first-response reviews, the Dow Jones index appeared to pan the speech with its steady decline through trading hours, losing 332 points on the day, with a dip, not a blip, in the minutes immediately following the address. Certainly Obama failed to inject the nation with a shot of instant, FDR-style consumer confidence. "The state of our economy calls for action, bold and swift," he said, but already his stimulus package is under attack from Keynesians for being far too little and from fiscal conservatives for being far too much. His appeals for a renewed spirit of community and mutual responsibility, though phrased more vividly than Clinton's in 1997 ("It is the firefighter's courage to storm a stairway full of smoke, but also a parent's willingness to nurture a child, that finally decides our fate"), were cast in the same communitarian mould as Clinton's, and had the unfortunate effect of reminding me, at any rate, of the dullest inaugural in living memory. It was this aspect of Obama's address that his aides hawked around the TV networks as its dominant theme: he would, they said, use the address to herald "a new era of responsibility". But that was a blind: Obama's real preoccupations lay elsewhere.

Next week, the speech will be pretty much forgotten, and people will scratch their heads to remember a single quote from it. Yet if (and it's a huge if) 2009 should eventually turn out to have been the date when the United States renounced the accumulated policies of the Bush years, regained an honourable place in the wider world and returned to the course of its "better history", then we'll reread Obama's inaugural and discover how subtly audacious he was being. It's already original - and not so much in spite of, but because of, its unoriginal language. It might, just conceivably, be seen as revolutionary.