Slipstreaming behind the annual rituals of sorrow and reverence for 9/11, George W Bush has decreed that, five days later, on the 16th, there is to be a further day of solemnities on which the nation will pray for the unnumbered victims of Hurricane Katrina. Prayers (like vacations) are the default mode for this president who knows how to chuckle and bow the head in the midst of disaster but not, when it counts, how to govern or to command. If you feel the prickly heat of politics, summon a hymn to make it go away; make accountability seem a blasphemy.

Thus has George Bush become the Archbishop of Washington even as his aura as lord protector slides into the putrid black lagoon, bobbing with cadavers and slick with oil, that has swallowed New Orleans. No doubt the born-again president is himself sincere about invoking the Almighty. But you can hear the muttered advice in the White House: Mr President, we were in trouble after 9/11; the unfortunate episode of the schoolroom, My Little Goat and all that. But do what you did then; set yourself once more at the centre of the nation; go to the epicentre of the horror and embrace its heroes; make yourself the country's patriotic invigorator and all may yet be well.

So this weekend it was predictable that the president would shamelessly invoke the spirit of 9/11 to cover his shamefully exposed rear end - "resolve of nation ... defend freedom ... rebuild wounded city ... care for our neighbours". But comparisons with 9/11 - the fourth anniversary of which was marked in New York yesterday - will only serve now to reinforce the differences between what the two calamities said about America, and especially about those entrusted with its government. The carnage of 9/11 generated an intense surge of patriotic solidarity, even with America's Babylon, a city scandalously and notoriously indifferent to Heartland values. This was because the mass murders had been committed by people who defined foreignness: theocratic nihilists who equated pluralist democracy with depravity. A hard-ass city supposedly abandoned to the most brutal forms of aggressive individualism (a fiction it liked to cultivate) showed instead the face of American mutualism as volunteers poured into the smouldering toxic crater. Blood and food donations piled up and a mayor disregarded his personal safety to be where he had to be, in the thick of the inferno; his daily press conferences astoundingly bullshit-free, unafraid of bearing bad news; treating his fellow-citizens, mirabile dictu, like grown-ups.

The rest of the country looked at Zoo York and, astoundingly, saw images and heard stories that made themselves feel good about being American: the flag of defiance flown by firemen amid the Gothic ruins; the countless tales of bravery and sacrifice among those trapped inside the towers. For all the horror, this could be made into a good epic of the American character. It was this redeeming sense of national community that protected the president from any kind of serious political scrutiny whenever he invoked 9/11 as the overwhelming reason for launching the invasion of Iraq. As John Kerry found to his cost, unexamined passion triumphed over reasoned argument. Bush won re-election simply by making debate a kind of treason; an offence against the entombed.

Out of the genuinely noble response to 9/11, then, came an unconscionable deceit. Out of the ignoble response to Katrina will come a salutary truth. For along with much of New Orleans, the hurricane has swept away, at last, the shameful American era of the fearfully buttoned lip. Television networks that have self-censored themselves into abject deference have not flinched from their responsibility to show corpses drifting in the water; lines of the forlorn and the abandoned sitting amid piles of garbage outside the Convention Centre; patients from Charity Hospital waiting in the broiling sun in vain for water and medical supplies; helicopters too frightened of armed looters to actually land, but throwing bottles of water down from their 20ft hover. Embarrassed by their ignorance of the cesspool that was the Convention Centre, members of the government protested that it was hard to know what was really going on "on the ground". All they had to do was to turn on the TV to find out.

Millions of ordinary Americans did. And what they saw, as so many of them have said, was the brutality, destitution, desperation and chaos of the Third World. Instead of instinctive solidarity and compassion, they have witnessed a descent into a Hobbesian state of nature; with Leviathan offering fly-by compassion, 30,000ft up, and then, once returned to the White House, broadcasting a defensive laundry list of deliveries, few of which showed up when and where they were needed. Instead of acts of mutual succour, there was the police force of Gretna, south of New Orleans, sealing off a bridge against incoming evacuees, and turning them back under threats of gunfire. Instead of a ubiquitous mayor with his finger on the pulse, and the guts to tell the truth, enter Michael Brown, a pathetically inadequate director of the Federal Emergency Management Agency, Fema, hounded from his 11-year tenure as supervisor of commissioners and stewards of the International Arabian Horse Association by legal proceedings. Instead of summarily firing "Brownie", the president ostentatiously congratulated him on camera for doing "a heck of a job".

Only on Friday, in an attempt at damage control, was the hapless Brown "recalled" to Washington, his position as Fema director intact.

And instead of an urban community of every conceivable race, religion and even class brought together by trauma, another kind of city, startlingly divided by race and fortune, has symbolised everything about America that makes its people uneasy, ashamed and, finally, perhaps lethally for the conservative ascendancy and its myths, angry. A faint but detectable whiff of mortality is steaming up, not just from the Louisiana mire, but from this Republican administration. Call me a cynic but is it entirely a coincidence that suddenly the great black hope of moderate Republicanism, Colin Powell, is everywhere, publicly repenting of his speech to the UN (and by implication damning those who supplied him with unreliable intelligence), and offering, unbidden, his own lament for the institutional meltdown that followed the breach of the levee. The administration is already thought of as a turkey and the turkey vultures are starting to wheel.

Historians ought not to be in the prophecy business but I'll venture this one: Katrina will be seen as a watershed in the public and political life of the US, because it has put back into play the profound question of American government. Ever since Ronald Reagan proclaimed that government was not the answer but the problem, conservatism has stigmatised public service as parasitically unpatriotic, an anomaly in the robust self-sufficiency of American life. For the most part, Democrats have been too supine, too embarrassed and too inarticulate to fight back with a coherent defence of the legitimacy of democratic government. Now, if ever, is their moment; not to revive the New Deal or the Great Society (though unapologetically preserving social security might be a start) but to stake a claim to being the party that delivers competent, humane, responsive government, the party of public trust.

For the most shocking difference between 9/11 and Katrina was in what might have been expected in the aftermath of disaster. For all the intelligence soundings, it was impossible to predict the ferocity, much less the timing, of the 9/11 attacks. But Katrina was the most anticipated catastrophe in modern American history. Perhaps the lowest point in Bush's abject performance last week was when he claimed that no one could have predicted the breach in the New Orleans levees, when report after report commissioned by him, not to mention a simulation just last year, had done precisely that. But he had cut the budget appropriation for maintaining flood defences by nearly 50%, so that for the first time in 37 years Louisiana was unable to supply the protection it knew it would need in the event of catastrophe. Likewise Fema, which under Bill Clinton had been a cabinet level agency reporting directly to the president, had under his successor been turned into a hiring opportunity for political hacks and cronies and disappeared into the lumbering behemoth of Homeland Security. It was Fema that failed the Gulf; Fema which failed to secure the delivery of food, water, ice and medical supplies desperately asked for by the Mayor of New Orleans; and it was the president and his government-averse administration that had made Fema a bad joke.

In the last election campaign George W Bush asked Americans to vote for him as the man who would best fulfil the most essential obligation of government: the impartial and vigilant protection of its citizens. Now the fraudulence of the claim has come back to haunt him, not in Baghdad but in the drowned counties of Louisiana. In the recoil, disgust and fury felt by millions of Americans at this abdication of responsibility, the president - notwithstanding his comically self-serving promise to lead an inquiry into the fiasco - will assuredly reap the whirlwind.