Just maybe the dreaded "October Surprise" has already happened. The phenomenon is part of US electoral lore, shorthand for some unscripted bombshell event that at the climax of the campaign throws the contest to one of the candidates. But in this volatile, close-fought and ever more bitter US election, the job may have been done - by a 1,000-page report bearing the distinctly unpromising title "Comprehensive Report by the Special Adviser to the DCI on Iraq's WMD."
In a sense, of course, Charles Duelfer, the man who for the past nine months has led the search for Saddam Hussein's non-existent weapons of mass destruction, did not deliver a surprise at all. Everyone knew there were no weapons - even George Tenet, the former director of Central Intelligence, who had commissioned the effort and who once boasted to President Bush of a "slam-dunk" case against Saddam. It had long been obvious that the pre-war intelligence was wildly off the mark - and equally obvious that the Bush administration had made matters worse still by shamelessly hyping that intelligence.
But the Duelfer report was devastating nonetheless. It is beyond all refutation. Not only did Saddam have no weapons; he got rid of them long ago, shortly after the 1991 Gulf War. Not only did weapons not exist - there were not even programmes for weapons, merely an unspecified "intention" on the part of Saddam to get back into the WMD business once United Nations sanctions had been lifted.
In short, there was no threat, either imminent or in the medium term. The policy of containment and UN inspections was working. The President's handlers had braced themselves for trouble, but even they were taken aback by the starkness of Mr Duelfer's conclusions. The report may have been one of those tipping points that decide a campaign, in retrospect the moment at which a president's credibility is fatally undermined.
For weeks, John Kerry has been arguing that Mr Bush and Dick Cheney, his Vice-President, live in a make-believe universe of their own, claiming that Saddam had been a major threat to world peace, and that all was going well in Iraq, despite the disorder and carnage on TV screens every night. After Mr Cheney had actually contended during his debate with John Edwards that the report had strengthened, not weakened the case for invasion, the Democrat ridiculed the President and Vice-President as "the last two people on the planet who won't face the truth about Iraq". It was the confident jibe of a candidate who senses the tide is turning his way.
In fact, the report that Mr Duelfer presented to the Senate Armed Services Committee on Wednesday was but the low point of a thoroughly dismal fortnight for Mr Bush and his disintegrating case for war against Iraq. It began when Paul Pillar, the top CIA officer for the Middle East and South Asia, let it be known that the agency had warned beforehand that an invasion would provoke rebellion in Iraq and a surge in sympathy for radical Islam.
A few days later, The New York Times published an exhaustive and widely noted account of the "aluminium tubes affair", showing how the Bush White House built up the Iraqi nuclear scare by maintaining Saddam was buying aluminium tubes to enrich uranium - despite being told by its own Department of Energy and a host of other experts that they were for perfectly legitimate artillery rockets.
Then yet another leaked CIA report from this once most leak-proof of administrations cast doubt on the links between Saddam and Musab al-Zarqawi, the Jordanian militant linked with al-Qa'ida who masterminded the kidnapping and execution of Western hostages, including the Briton Ken Bigley. That finding tore another chunk from the administration's rationale for war, that the former Iraqi regime was in cahoots with Osama bin Laden.
These attacks, emanating from America's flagship liberal newspaper and a CIA now conducting a guerrilla war of its own against the White House, might have been expected. Not so, however, the damning comments from some of the President's closest advisers on Iraq, and the even more damning nature of the Duelfer report. First, Defence Secretary Donald Rumsfeld, an architect of the war, blithely declared that he for one had never seen "strong, hard evidence" of connections between Saddam and al-Qa'ida. Then Paul Bremer, until June America's all-powerful proconsul in Baghdad, told the same group of insurance brokers addressed by Cherie Blair in West Virginia that the administration had ignored his pleas to send more troops to Iraq immediately after the fall of Baghdad. This failure, he said, had allowed the violence and lawlessness that plague the country now to take root. Like Mr Rumsfeld, he claimed his remarks had been taken out of context, but the damage had been done.
Then came the considered judgement of the Iraq Survey Group, involving 1,700 inspectors and $900m of government money (incidentally almost as much has actually been spent on the post-war physical reconstruction of Iraq). The White House had no answer, other than to parrot the weary line that it had received the same intelligence as its Democratic critics on Capitol Hill (which conveniently ignores the fact that it was the White House itself that made sure that intelligence was presented in the most damaging possible way). In any case, Mr Bush averred, Saddam might have some day become a threat again, and the world was a better place without him.
But there was no arguing with the facts on the ground. Jay Rockefeller, the senior Democrat on the Senate Intelligence Committee, is a mild and courteous man, who voted for the war in October 2002. But last week he could not conceal his anger at the deception. "Unfortunately, there is no way to spin the fact that we have occupied Iraq now for 18 months and no WMD have been found. In short, we invaded a country, thousands of people have died, and Iraq never posed a grave or growing danger." Sanctions had "decimated Iraq's economy, infrastructure and ultimately, its ability to wage war and threaten its neighbours." True, Mr Bush put up a better performance in the St Louis "town hall" debate on Friday night than in his first confrontation with Mr Kerry eight days earlier - he could hardly have done worse. Gone was the testiness and scowling, and the hugely damaging impression he had left in Miami (just as his father had in his 1992 debates with Bill Clinton) that it was beneath the presidential dignity to discuss policy with a mere Senator from Massachusetts. But whenever Iraq came up, he was forced on to the defensive.
The President sharpened his rhetoric too. He positively dripped with contempt as he accused his opponent of "coming down firmly on every side of the Iraq war" and of advocating policies that would weaken America and make the world more dangerous. But he now has a problem. The harsher his attacks, the more he runs the risk of alienating voters. After all he, not Mr Kerry, is the one who is supposed to look presidential. His very ferocity, moreover, could reflect an inner fear - a gnawing realisation that an election that seemed almost in the bag before the damaging news about Iraq, might actually be lost.
In Pennsylvania last week, Mr Bush wheeled out a new stump speech reflecting his more aggressive approach to Mr Kerry. "But that was a sign of real worry," says Norman Ornstein, a political scholar at the American Enterprise Institute. "Their aim is to scare their base to death. Whatever else, this election will see a level of acrimony and negativity that we've never seen before."
The tactic also turns on its head the pattern of presidential campaigns past. Normally an election involving an incumbent is a referendum on his performance over the past four years. This time Karl Rove, Mr Bush's master strategist, has - with considerable success thus far - tried to frame the campaign as a referendum on the challenger, and his fitness to be commander-in-chief. "They seem to be banking on voters taking a 'better the devil you know than the devil you don't' approach," Mr Ornstein says.
But the strategy may be nearing the end of its useful life. Only a certain number of times can you call a man a flip-flopper. And then there is the small matter of the facts. The Duelfer report was just one uncomfortable collision of Bush campaign rhetoric and reality. Hours before the showdown in St Louis came figures showing that the economic recovery trumpeted by the White House had created far fewer jobs than expected, barely half the number needed to keep up with a growing workforce, let alone bring people without jobs back into work.
This is now an election on a knife-edge, a campaign growing angrier and more negative by the day. Mr Kerry's strong performance in Miami erased the lead the President had built up after the Republican convention, bringing the race back to a statistical dead heat. The initial verdict on St Louis was that the second debate had been a tie. But according to local polls, the Democrat last week nosed back into the lead in some important battleground states, and for the second straight day the authoritative Hotline survey showed the challenger leading Mr Bush on Friday in terms of electoral college votes, by 245 to 218 (with 270 needed to win, and six states judged too close to call). Mr Bush's trump card remains his perceived strength as a leader in time of terror, and as the candidate best able to keep the country safe. But on that front, too, Mr Kerry is narrowing the gap.
Turnout is another crucial variable. Last time it barely topped 50 per cent. Never though has the country been as polarised as now, and never has public interest been higher. The first debate drew 63 million viewers, half as many again as watched Mr Bush's first encounter with Al Gore in 2000. The audience for the vice-presidential debate was 50 per cent up on its counterpart four years ago.
Both parties have mounted unprecedented voter-registration drives in swing states. The signs are the Democrats have been more successful, signing up new black and minority voters in particular. But then again, how many of them will actually go to the polls on 2 November? Meanwhile, evangelical Christians (overwhelmingly Republican and presumably including many of the four million who, according to Rove, did not vote in 2000) are mobilised as never before.
But an outside event, an "October surprise", could transform everything.
Usually American elections are decided on bread-and-butter domestic issues, and the debates could yet unexpectedly provide a defining moment. But not since Lyndon Johnson threw in the towel in 1968 over Vietnam has an election been so at the mercy of outside events as this one.
The Muslim holy month of Ramadan is at hand. A surge in US casualties in Iraq could damage the Republicans, as could a terrorist attack on the homeland - though does al-Qa'ida want the President to win or lose? If the latter, then an attack might be counterproductive by causing Americans to rally round the President. Another Republican fear is a sudden surge in oil, and hence petrol, prices. Chaos at the Afghan elections this weekend would also hurt the President.
But Democrats are no less alarmed. Things might actually start going better in Iraq. A successful vote in Afghanistan could lessen voters' doubts about Mr Bush's broader vision of bringing democracy to Iraq.
Their greatest dread, however, is the capture or killing of Osama bin Laden just in time for polling day. The more paranoid among them believe that the al-Qa'ida leader is already in some secret US detention centre, to be unveiled at the perfect moment, just before polling day.
All or none of the above may occur. But none of them may have as great an impact as the Iraq policy embarrassments that befell Mr Bush last week, culminating in the Duelfer report. "I'm an analyst, but I realise I'm in a political world right now," its author told the Armed Services Committee, in what must be a candidate for understatement of the year. Right now, that political world could be turning against Mr Bush, perhaps irrevocably.