Despite such volatility, opinion polls point to the Democrats as likely to take over the House of Representatives, giving them an outside chance of regaining control in the Senate.

Q What is at stake?

A The Republicans’ 12-year domination of Congress — the Houses of Representatives and the Senate — could be ended. Although Mr Bush cannot be ousted before his second term expires in January 2009, Democrats have tried to turn these elections into a national referendum on both him and the war in Iraq.

Q What is the House of Representatives?

A The junior and more partisan chamber — the “house of the people” — where all spending Bills are initiated. Each state is represented in proportion to its population and all 435 members must seek re-election every two years. The Democrats need to make 15 net gains to get control.

Q What is the Senate?

A Designed to be less susceptible to shifts in public opinion, it has the power to block treaties and Administration appointments. Each of the 50 states has two Senators and members are elected on a six-year cycle. On Tuesday, 33 seats are being contested — 15 of them held by the Republicans of which the Democrats must win six to regain control.

Q What happens if the Democrats take control of the House?

A They have promised legislation within 100 days to raise the minimum wage, cut student loans and clean up congressional sleaze. Beyond that, some want revenge for what the Republicans did to Bill Clinton in 1994 — when they buried the White House under a blizzard of subpoenas and, ultimately, impeached the President. But Nancy Pelosi, who would become the Democrats’ Speaker of the House, has ruled out impeaching Mr Bush as “a waste of time”. She does not want “any out-of-control investigations” on matters such as the allegedly illegal practice of domestic wiretapping.

Although restraining committee chairmen may be easier said than done, senior Democrats have promised to focus on oversight, especially “waste, fraud and abuse” in Iraq. Such inquiries, they believe, will discredit arguments for continuing in Iraq faster than any retrospective inquiry into prewar intelligence.

Symbolic votes on setting a timetable for withdrawing troops, or even attempts to set conditions on military spending Bills for redeploying soldiers, would mean clashes between the executive and legislative arms of government. But Mr Bush could find it easier to get a deal on giving legal status to millions of undocumented immigrant workers, a plan that has been approved by the Senate but blocked by conservative Republicans in the House.()

Q What happens if the Democrats win control of the Senate as well?

A Certainly more investigations, inquiries and subpoenas. They could also block another Bush nomination to the US Supreme Court and, thereby, deny the President the chance of creating an enduring legacy distinct from Iraq. Mr Bush has so far made two such life-time appointments — but needs one more to create a conservative-leaning majority of the nine-judge body.

Incoming Democrats elected on protectionist platforms will jeopardise further hopes of a breakthrough in global trade talks. With his domestic agenda blocked on Capitol Hill, Mr Bush — with eyes on his legacy — could be expected to spend his final two years focusing on foreign affairs, particularly the Middle East.

Q What if the Democrats win neither?

A There will still be change in Washington: even the most optimistic Republicans expect the Democrats to make gains and Mr Bush will be under pressure to heed the message of the electorate. For instance, the Administration is likely to navigate a significantly different course on Iraq when an inquiry led by James Baker, the former Secretary of State, publishes its report after the election.

Q Are these elections about anything else?

A Yes. The Republicans may lose eight state governorships and power in a number of state legislatures. In addition, Democrats hope that local referendums on raising the minimum wage in six key states will increase turnout among their natural supporters.

Q What does this mean for the next presidential contest in 2008?

A If the Democrats win, they will have a record in power that can be used for, or against, them. Some Republicans believe that a House run by a “liberal Democrat” like Ms Pelosi will help to discredit their opponents in 2008.

Alternatively, if the Democrats fail to make a breakthrough in this election they may despair of ever doing so and revert to old habits of ripping themselves apart.

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