Americans go to the polls today. The only question is the extent of the reversal that the Republicans will sustain, writes Denis Staunton in Washington.
Unless almost every opinion poll, political analyst and party strategist is wildly mistaken, Americans will wake up tomorrow to a changed political landscape, with the Democrats in control of the House of Representatives for the first time in 12 years.
The Democrats need to gain just 15 seats for a majority in the House, where all 435 seats are up for election, but they are widely expected to pick up at least 20 and possibly as many as 35. The party that wins in the House usually takes the Senate too, but this year's elections could be different as Democrats struggle to gain the six seats they need out of 33 up for election.
A big Democratic wave could, however, give the party a narrow Senate majority and Democrats are also likely to make gains among the 36 gubernatorial elections held today.
Nobody in either party doubts that this election will be characterised by Republican losses and Democratic gains: the only dispute is over the extent of the swing towards the Democrats.
The clearest evidence of the Republicans' defensiveness is president George Bush's recent itinerary, which took him to states like Nebraska, which he won by huge margins in 2000 and 2004. Now these seats are in danger and Mr Bush has been trying to rally the Republican's support base with warnings that a Democratic-run Congress will increase taxes and give up on the "war on terror".
For most Republican candidates, however, Mr Bush is a liability; his picture is much more likely to appear in Democratic campaign ads and literature. In Florida yesterday, for example, the Republican candidate for governor found an urgent reason to be elsewhere when the president came to campaign.
The vice-president, Dick Cheney, has disappeared from the campaign altogether for the last two days to go hunting for the first time since he accidentally shot his friend Harry Whittington in February.
A Democratic victory today would owe much to the president's unpopularity and to popular revulsion at the financial and sexual scandals that have dogged Republicans in Congress.
The administration's slow and chaotic response to Hurricane Katrina last year dealt a serious blow to their reputation for competence. The failure of the Republican-controlled Congress to reform immigration, overhaul the state pension system and streamline healthcare for the poor and elderly has left fiscal conservatives unhappy.
Evangelical Christians are disappointed that despite controlling both the executive and the legislature, Republicans have failed to advance the conservative agenda on issues such as abortion and gay rights.
If Republicans lose control of Congress, however, it will be above all on account of the Iraq war, which two out of three Americans now believe was not worth fighting and has not left the US safer.
Travelling through the US in recent months, it was hard to find a single voter who approved of the administration's conduct of the war.
At an American Legion hall in Pennsylvania, veterans expressed support for Mr Bush as commander in chief but excoriated defence secretary Donald Rumsfeld. In rural Minnesota, the parents of a young soldier serving in Iraq could not bring themselves to say that the war was worthwhile.
In New York, relatives of September 11th victims said they supported the troops in Iraq but would not say that the war had helped to prevent another attack on the US.
Even among the neo-conservatives in Washington who were the loudest cheerleaders for war, most now admit that the occupation was bungled and many acknowledge that the invasion was a mistake.
Iraq has cost almost 3,000 American lives and killed an unknown number of Iraqis, costing more than $300 billion. Around Washington, returned wounded soldiers are an increasingly common sight, many of them amputees just released from the Walter Reed army medical centre or the naval hospital in Bethesda.
In other US cities, a small but growing number of young veterans wander the streets homeless, often suffering from depression, anxiety or post-traumatic stress disorder.
Before he disappeared on his hunting trip, Mr Cheney told ABC News that a Democratic victory today would make no difference to the administration's policy on Iraq.
"It may not be popular with the public," he said. "It doesn't matter, in the sense that we have to continue the mission and do what we think is right. And that's exactly what we're doing. We're not running for office, we're doing what we think is right."
The president is responsible for foreign policy and Democrats have made clear they will not pull the plug on Iraq by withdrawing funding for the military operation there.
A Democratic-controlled House would, however, subject the administration's Iraq policy to greater oversight, holding hearings into the use of intelligence before the war, the conduct of the campaign and the occupation and the award of contracts without competition to firms like Halliburton.
If Democrats take the Senate, they will be in a stronger position to present to the public an alternative approach to Iraq. Even if the Republicans retain control, Senate armed services committee chairman John Warner has promised to hold hearings into Iraq next week, signalling that the current policy needs review.
Any change in the administration's Iraq policy will probably follow the publication of a report by the Iraq Study Group, a bipartisan committee chaired by former secretary of state James Baker and former congressman Lee Hamilton, which has the blessing of both Congress and the White House.
After his election victory in 2004, Mr Bush declared that he had a strong, fresh mandate to implement his political agenda. "Let me put it to you this way," he said then. "I earned capital in the campaign, political capital, and now I intend to spend it."
A Democratic victory would confirm that the president's political capital is now exhausted and that his only hope of achieving any of his domestic political goals lies in working with the Democrats.
As governor of Texas, Mr Bush showed a capacity for compromise and Democrats may be willing to work with him on such issues as immigration reform, education and improving homeland security.
The loss of Republican control over Congress could have far-reaching consequences for the conservative coalition that has sustained Mr Bush in power but is now increasingly fractious. A sweeping Democratic victory could even mark the beginning of a realignment of American politics, with a renewal of confidence in the role of government at home and after 2008 a return to a more traditional, multi-lateral approach to foreign policy.