What if? For students of history, these are arguably the two most intriguing words in the English language. What if Hitler had not invaded Russia? What if massive storms in the Channel and North Sea had not wrecked the Spanish Armada?

And, for that matter, what if Robert Kennedy had not been assassinated in 1968 – or if Ronald Reagan had been assassinated in March 1981, when he had been in office a bare two months? And how different things might be today had a couple of thousand voters in Palm Beach County, Florida, not accidentally cast their presidential votes in November 2000 for Pat Buchanan instead of Al Gore, thanks to the poorly designed butterfly ballot.

Eight years on, another presidential election today reaches its moment of truth. At this stage, the final 2008 match-up has an air of inevitability about it. It is easy to believe, in hindsight, that the enthusiasm generated by Barack Obama, his superior campaign organisation and the vast sums of money he raised, guaranteed he would win the nomination. As for Republican primary voters, did they not quickly realise that John McCain was the only candidate who offered a chance of their party hanging on to the White House?

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But in this campaign, as in almost every recent US election, nothing was set in stone. Hillary Clinton was the prohibitive early favourite for the Democratic nomination; in summer 2007, months before a vote was cast, McCain had been given up for dead. In all these elections, there were candidates who ran and lost, and others who might have run, but chose not to.

In 2008, the great unknowable is: what if Hillary had won the nomination – and almost certainly occupied the same position of overwhelming favourite that Obama does this morning?

What sort of President would she be? More competent, perhaps, than her husband, but less inspirational, and with an ear less finely tuned to politics' subtle flows. Probably we shall never know – though in the unlikely event that Obama loses today, she will be at the front of the queue for 2012. But had she gone all the way, the American presidency might have become the preserve of two competing, alternating dynasties – with one Jeb Bush waiting in the wings when Hillary's time was up.

Which leads to another "what if?". Had Jeb not been defeated in his bid for the Florida governorship in 1994 (the year George W won in Texas), he, rather than his elder brother, might have been the Republican nominee who faced Gore in 2000. Don't forget that among the Bushes, Jeb, not George, was always considered the son most likely to succeed in politics. And had Jeb made it, the US might have been spared the Iraq war, Dick Cheney and Donald Rumsfeld, and the global surge of anti-Americanism.

That, it may be said with certainty, would have been the case had not those butterfly ballots intervened. After September 11, a President Gore would have surely attacked the Taliban, al-Qa'ida's protectors, in Afghanistan. But he would not have been ordering plans for the invasion of Iraq while the World Trade Center still smouldered. He would have taken global warming seriously. He wouldn't have antagonised Europe by trashing the Kyoto treaty.

But in terms of what might have been, Gore doesn't hold a candle to the sons of old Joe Kennedy. Had he lived, JFK would almost certainly have won a second term in 1964, and perhaps spared the US the agony of Vietnam. If Bobby Kennedy had not been murdered four years later, he would almost certainly have been the 1968 Democratic nominee. Quite possibly, he would have become President, with an unequivocal promise to end that war. In the end, Vice-President Hubert Humphrey almost snatched victory from Richard Nixon, despite his identification with Lyndon Johnson's Vietnam policy.

And then there is Teddy. But for Chappaquiddick, he would surely have been the Democratic nominee in 1972, 1976 or 1980, perhaps even later. Probably, he would have won the White House. What sort of President would he have been? Liberal to be sure, but perhaps not as effective as he has been in the Senate.

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Thus far no businessman has won the presidency, but since 1980 two have had a serious shot. Ross Perot came closest, in 1992, when at one point he even led both incumbent George HW Bush and his Democratic party challenger, Bill Clinton, in the polls.

Perot was a flawed candidate. He was prejudiced, hectoring and dictatorial. He probably would have made a terrible President. Accustomed to having their orders in the boardroom instantly carried out, business executives rarely adapt to the meandering ways of politics. But every so often, America yearns for a non-partisan, can-do figure. In 1992, amid a sluggish economy and soaring deficits, Perot won almost 20 per cent of the popular vote despite his manifest shortcomings. And had Perot not run, the elder Bush might well have defeated Clinton – another tantalising "what if?".

A less flawed businessman-politician was Lee Iacocca, the folksy boss of Chrysler who saved the US carmaker from bankruptcy, and at one point was running ahead of Vice-President George HW Bush in putative match-ups for the 1988 Republican nomination. In those days Iacocca was a cult hero, but he realised that running a country was more difficult than engineering a corporate turn-around. More recently, Michael Bloomberg, the mayor of New York, another executive with bipartisan appeal, mulled a 2008 White House run, before deciding to give it a pass.

Where businessmen rarely venture, however, generals do. Like businessmen, generals are seen to be above the partisan fray, and they are usually war heroes to boot. Dwight Eisenhower had never been elected to anything before he won the presidency, virtually by popular acclaim, in 1952. He was a Republican, but he could have run as a Democrat and the result would have been the same.

Ditto Colin Powell – except that Powell didn't run. In the mid-1990s, the former chairman of the joint chiefs and national security adviser seemed an odds- on bet to become America's first African-American President. But a year before the 1996 election, Powell took himself out of consideration, largely because of his wife's fear he might be assassinated.

Events since – notably Powell's support for the Iraq invasion – have suggested he probably wasn't ideal for the job. Ultimately, Powell the soldier was happier executing the orders of others than of giving them himself. But he, too, is a might-have-been, another "what if?" in a US presidential history full of them.