Karl Rove, the grand wizard of strategy known as 'Bush's brain', seemed close to realising his ambition to create a Republican hegemony that would last for a generation. He had a dream of turning America into a one-party state and it was a dream that looked like becoming all too real. A country founded on the idea of the separation of powers has rarely witnessed such a concentration of might in the hands of one party.

Such hubris is always the midwife to nemesis. Suddenly that Republican domination is beginning to crack. This autumn the tectonic plates of American politics are beginning to shift under the feet of President Bush and an increasingly desperate Republican party. When I spoke to Stan Greenberg, the hugely experienced political consultant for the Democrats, he predicted an 'earthquake' in the mid-term elections for a third of the Senate and all of the House of Representatives. Even more tellingly, the Republicans themselves sound very scared that angry voters are about to punish them with a thrashing. Thomas Davis, a Virginian Congressman who is one of his party's most senior strategists, talks about the Republicans losing as many as 30 seats in the House, which would put that half of Congress into the hands of the Democrats for the first time in 12 years.

Some say it is the war. Some say it is the money. Some say it is the sex. Actually, it's all three, a triple-whammy of reasons for Americans to express their disgust with how they are being governed.

Let's start with Iraq. Any American with a television set and an IQ above room temperature has known for a long time that Iraq is far from becoming the pacified, liberal democracy that was promised in the original prospectus for the war. Most Americans were nevertheless prepared to tolerate the mounting carnage so long as they could believe that the ultimate outcome would be positive. There has been a big turn in the mood about the war in the past fortnight. John Warner, the Republican who chairs the Senate's armed services committee, came back from a visit to American troops in Iraq to warn that there had to be 'a change of course'. A commission chaired by James Baker, Secretary of State when Dubya's father was in the White House, is about to publish a report calling for a major recasting of strategy.

These rock-ribbed Republicans cannot be dismissed with the usual White House line that anyone who asks awkward questions about Iraq is an unpatriotic appeaser and fellow traveller of Osama bin Laden. Soaring up the bestseller lists is Bob Woodward's account of a dysfunctional administration presided over by a wilfully uninquiring Commander-in-Chief who will never acknowledge the scale of the blunders committed in Iraq.

President Bush has again tried to use national security as his trump card in this election. The terror of terror worked for the Republicans in 2002 and again in 2004. It is not working this time. The opinion polls all agree: a majority of Americans now feel that Iraq is getting worse, and that the war was a mistake which has left them less secure.

They still see Bush as a 'War President'. The difference now is that they see him losing his wars. The United States has invaded Iraq and not found any weapons of mass destruction while North Korea is acquiring the nuclear bombs which George Bush once pledged he'd prevent them from having. At a news conference at the White House, the President talked big about Kim Jong-Il but carried a small stick. The world's soi-disant hyperpower is reduced to suggesting that China should do something about it.

What is most alarming people, including senior members of Bush's own administration, is how the crisis over North Korea plays into the threat of a nuclear-tipped Iran. The more helpless that America looks in relation to North Korea, the more emboldened the Iranians will feel about defiantly pursuing their ambitions to join the nuclear club. The Bush presidency has expended squillions of dollars on warfare and military hardware. So much treasure and so much blood and Americans are left with a growing dread that they have ended up weaker in the world.

Then there's the sex. While his party shamelessly fanned homophobia to ramp up its vote, a gay Republican congressman was making advances to teenage male interns. Congressman Mark Foley has resigned his Palm Beach seat since his dirty computer messages were exposed and the fall-out from his cyber-stalking of teenagers could cost other Republicans their places in Congress.

As is so often the case, the Nixon rule of scandal applies. It is not so much the crime as the cover-up that has done the most damage. There has been a corrosive drip of accusations that the party leadership in Congress ignored warnings about Foley's behaviour. The Republicans are reeling from the impression that the self-appointed moral daddies of America harboured a sexual predator.

When I spoke to Andy Card, who for five years was Chief of Staff to President Bush, he calculated that the election would ultimately come down to which side could mobilise more of its supporters in the last 72 hours. The Rove vote machine has been heavily reliant on evangelical Christians, precisely the group most repelled by what it sees as moral degeneracy on Capitol Hill.

And then there is the money. A rising stench of corruption surrounds the Republicans. The scale of the kickbacks made to politicians by Jack Abramoff, the convicted lobbyist, are awesome even by the standards of American bribery scandals. A defining theme of the Bush era has been Republicans who preach fiscal abstinence while practising recklessly unprotected spending. The surplus inherited from Bill Clinton has been blown and turned into a staggering deficit. The richest and most powerful country on the planet is now in the strange and dangerous place of being hugely indebted to the rest of the world. Put it all together - and I get the sense that Americans are finally putting it all together - and the Republicans look like a party that is jeopardising their nation's moral, strategic and financial future.

You have to say, it couldn't have happened to a nicer bunch of people. Lynn Westmoreland is running for re-election as a Republican congressman in Georgia. His sole legislative initiative has been to press a bill requiring that the Ten Commandments be displayed in the House and the Senate. He then had to confess on television: 'I can't name them all.' In fact, he could barely name three of the commandments that he was so keen on. Voters in Iowa have on offer the Republican Steve King. He wants to keep out illegal immigrants by constructing a 700-mile wall along the border with Mexico. Better still, he built his own model of this 'Tortilla Curtain' out of cardboard and wire which he demonstrated to Congress in Blue Peter fashion. That is outdone in the crazy stakes by the Texan Republican Sam Johnson who offered personally to fly an F-15 to nuke Syria. Afterwards, he said he was: 'Kinda joking.' Don't you love the 'kinda'.

Don Sherwood, a Pennsylvania Republican, is famous for paying an undisclosed sum to his former mistress, who had accused him of repeated assaults, to settle her lawsuit against him. He has been forced to broadcast campaign ads denying that he tried to choke her. Down in Florida, Katherine Harris, who achieved world notoriety over the hanging chads which gave Bush the White House in the first place, is running for the Senate. According to her: 'God is the one who chooses our rulers.' Mmm. If the Great Returning Officer really does bother himself with deciding elections, then God must be mighty pissed with America to have chosen rulers like these.

A slew of recent opinion polls shows support for the Republicans plummeting and the Democrats gaining what should be a decisive edge. Gallup gives the Democrats a lead of more than 20 per cent among likely voters. Given such a toxic blend of policy failure abroad, financial and sexual scandals at home, compounded by discontent about the economy, in most democracies the governing party would be expecting a total meltdown. The purgative mechanism of the ballot would do its necessary work to kick the scoundrels out.

And yet you have to be a little cautious about predicting that the Republicans will suffer the sort of wipe-out that natural justice says they deserve. America is in a febrile state. There are three weeks left before election day and the polls have yo-yoed depending on the sleaze or terror headline of the hour.

While America's mood is volatile, its democracy is becoming atrophied. And by design. The gerrymandering of seats to permanently fix their political complexion has made it extraordinarily difficult to dislodge incumbents.

The story of this election is one of Republican collapse rather than any great enthusiasm for the Democrats. They don't have a clear message delivered by a popular and plausible leader. One of the Democrat's best hopes for the presidency - Mark Warner of Virginia - has just backed out of the race for 2008. It is in the nature of the American system that the executive can speak with a single voice - that of the President - while the opposition talks in a cacophony of tongues.

A senior member of the Clinton cabinet put it to me like this: 'The Democrats don't have one spokesman. They have 10 spokesmen.' There is no such thing as the Shadow President. If ever there was a country in need of a leader of the opposition, it is the United States today.

Even in the absence of one, George Bush faces a bleak closing chapter of his presidency. The Democrats need only gain control of one house to start launching investigations into 9/11, the Iraq war and its searing aftermath, the financial scandals, the sexual scandals - you name it, they can subject the White House to torture-by-inquiry. A Democrat majority in the House will almost certainly give the chairmanship of the judiciary committee to John Conyers who has previously called for the impeachment of the President.

George Bush is set to spend his last two years in the White House besieged by searing probes into his presidency. That would be a fitting fate for a President famous for his unwillingness or inability to focus on detail and his lack of curiosity about the consequences of his own decisions. The neck lock will then be on George W Bush.