Of the many theories purporting to explain the increasingly unpredictable contours of the US primary election season, the most obvious has not yet become the conventional wisdom that it richly deserves to be. Perhaps it’s the simplicity of the point that has submerged it. Perhaps the subject has become such a crashing bore that we all simply prefer to move on. But that doesn’t make it any the less true. This election has been crafted by the man who isn’t in it. This is an election about George W Bush.

Most presidential elections, to be sure, are about the last one. Young, electrifying JFK was the antidote to arguably the best and certainly the most tedious president of the last century, Dwight Eisenhower. Jimmy Carter was a purist rebuke to the sordidness of Watergate. Ronald Reagan in turn provided the stylistic pomp and ideological clarity that Carter clearly lacked. George Bush Sr was a “kinder, gentler” Yankier version of Reagan conservatism. Bill Clinton was the clued-in, lower-class hipster to replace the out-of-it patrician pensioner of the first Bush. W, in turn, was the plain-spoken, hedgehog-rather-than-fox antidote to the wily, slippery, verbose Bubba Bill Clinton.

And now we have the three potential Bush replacements: John McCain, the man who ran against him in 2000, voted against his tax cuts, excoriated his torture policy and assailed his Iraq occupation; Hillary, the wife of the man Bush succeeded and who beat his daddy; and Barack Obama, a young, charismatic JFK-liberal whose eloquence and erudition are almost textbook negatives of Bush’s folksy, faux-ignorant charm. It all makes a little more sense now, doesn’t it?

But Bush has empowered his nemeses even more than most presidents. Partly this was a function of the absence of an obvious successor. Unlike most presidents, he had no vice-president to succeed him. Dick Cheney never intended to run for election in his own right (and even seemed somewhat affronted that he had to parade himself before the voters for approval in 2004). The most obvious appointed successor – the accomplished former governor of Florida, Jeb – was a Bush too far even for America’s current dynasticism.

And a Bush clone simply wasn’t available. The president’s uncanny ability to persuade the business elites he was one of them, to appeal to evangelicals by actually being one of them and to corral national security voters on the war was, it turns out, sui generis.

The business elites liked Mormon Mitt Romney enough – but the evangelicals didn’t. The Southern Christianist right loved Mike Huckabee. And the hawks championed McCain, but everyone else mistrusted him. McCain, to be sure, won the nomination last week. But it would be more accurate to say that everyone else lost it. McCain failed to secure a clear majority in the primaries and caucuses. In fact, he’s the only successful candidate I can recall who had to spend the day of his final victory apol-ogising to party activists for winning.

And what, in the end, was the positive Republican reason to vote for McCain? To my mind, it came down to government spending and the war in Iraq. On both core subjects, McCain became the Republican antidote to Bush, without forcing the Republican base of the party to actively repudiate the sitting president. This was a very hard trick to pull off, and McCain hasn’t been given enough credit for managing it.

Bush’s greatest domestic conservative failure has been fiscal. The explosion of discretionary and entitlement spending on his watch would make a left-liberal blush. And one of the core, consistent principles of McCain has been fiscal rectitude. Voting for McCain now is a way for conservatives to renounce the fiscal reckless-ness of the Bush era. And McCain was careful not to blame the president publicly for the mess.

Iraq is a more complicated picture. It is now extremely hard – and I mean extremely – to defend the decision to go to war in 2003. In hindsight, it looks like one of the biggest miscalcu-lations in the history of American foreign policy (and yes, I was for it). But McCain has brilliantly been able to change the subject from this debacle by arguing forcefully that the project is still defensible in principle, if botched brutally in practice.

His brave criticism of Donald Rums-feld and the occupation nonstrategy in 2004 led to the surge; and the surge’s surprising tactical success in bringing a raging civil war down to 2005 levels of murderousness has enabled him to gain just enough credibility to run a national security campaign against the Democrats.

And so he manages both to rebuke Bush while rescuing his most troubled legacy. I’m sceptical he can do it if he gets elected. But his is the only position on the war that both pleases the Republican base and retains even a semblance of credibility among the public at large. For good measure, he opposes the torture policy that many conservatives privately feel ashamed about.

Bush is the reason, in other words, that this unlikely maverick has become the Republican nominee. And Bush is also the reason, I would argue, that Hillary Clinton’s meticulously planned coronation as the next Democratic nominee came unstuck.

It came unstuck because the depth of the Democrats’ disgust with Bush required more than just partisan revenge. And in the glare of the campaign, the Clintons began to represent for many Democrats the kind of politics that Bush himself had mastered. They remembered that before Karl Rove there had been Dick Morris: political consultants skilled at dividing and polarising electorates to get their candidate a 51% victory. Would reelecting the Clintons be in some way an endorsement of continuing Bush-style politics?

If Bush had not so enraged and dispirited liberals, Clinton would have been fine as the next career politician running their machine. But Bush had become for this generation of Democrats what Nixon had become for a previous generation. And they wanted a revolution against him and all he represented. They wanted someone who had clearly opposed the Iraq war in the first place and would not foment a new one against Iran. They wanted someone who wouldn’t require translation by Washington professionals – but could instead inspire and rally the broader public.

The demoralisation of the Bush era made possible the emotional and social forces that have combined to create the Obama movement. Hurricane Katrina and the terrible treatment of poor blacks in New Orleans made a black man almost necessary. Abu Ghraib made a man of integrity important. And the stain that many liberal and independent Americans felt the Bush era had left required a much stronger astringent than careful, focus-grouped, split-the-difference Clintonism.

And that’s why the Republicans realised that up against the transformative power of Obama, they had to risk a move to the centre or face obliteration. Without Obama, I doubt that McCain would have emerged. But up against a clear, fresh, inspiring character, the Republicans couldn’t run a mere regional candidate like Huckabee or a phoney Bush composite like Romney. They needed a Republican who could appeal to the independents who were rushing to the Obama primaries and caucuses. And so Bush made Obama possible and Obama made McCain necessary.

If, as is still narrowly probable, the Clintons still strong-arm and bully their way to the Democratic nomination, something even more unexpected may happen. The conservative repudiation of Bush may finally beat the liberal version. History has its ironies. With McCain, this campaign may just have produced its biggest one.

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