Two days before the November 7 vote, dozens of congressional races were too close to call and two national public opinion surveys gave Democrats an edge but suggested a late burst of energy among Republicans.

Bush, weakened by the unpopular war in Iraq that may cost Republicans control of the Congress, seized on the death sentence imposed on Saddam Hussein as a hopeful sign of flourishing democracy in the violence-wracked country.

"History will record today's judgment as an important achievement on the path to a free and just and unified society," he said, vowing to "support Iraq's unity government as it works to bring peace to its great country."

White House spokesman Tony Snow dismissed as "preposterous" any suggestion that it manipulated the trial for maximum electoral advantage and said it was "absolute proof" that Iraq now possessed an "independent judiciary."

Democrats hailed the sentence against Saddam but warned it would not solve the challenges US forces face in Iraq, which was plagued by a raging insurgency and escalating sectarian strife that has fed a rising US death toll.

"Justice for the Iraqi people was finally served today," but "Iraq is in the middle of a civil war and growing sectarian violence will be an even greater concern following this verdict," said Democratic party chief Howard Dean.

The Democrats needed a net 15 seats out of the total 435 in play on Tuesday to control the House. They hoped for a gain of six Senate seats out of the 33 at stake that would give them the edge in the 100-member upper chamber.

A Democratic victory would dramatically reshape the political landscape for Bush's final two years in office and the 2008 election to choose his successor.

While expert forecasts of a Democratic victory in at least one and possibly both chambers have heartened the opposition party, two new national polls showed their once double-digit advantage sharply narrowed in recent weeks.

The Washington Post and ABC television showed likely voters favouring Democrats to Republicans by a 51 percent to 45 percent edge, with a margin of error of plus or minus three percentage points. In an October 22 survey, the Democrats had led 55-41. And a poll by the non-partisan Pew Research Centre that specialises in public opinion found a 47-43 lead for Democrats among likely voters, down from 50-39 two weeks ago. The margin of error was three percentage points.

Bush was campaigning in Nebraska and Kansas as part of a political rescue mission for embattled Republican candidates in 10 states. He was to cast his own ballot in his adoptive home state of Texas on Tuesday.

The US president has been fiercely attacking Democrats, accusing them of plotting to raise taxes and charging that they cannot be trusted to manage Iraq or the war on terrorism he launched after the September 11, 2001 attacks.

The Washington Post/ABC poll still had bleak news for Bush, with his approval rating at a dismal 40 percent and almost twice as many registered voters saying they would cast their ballot as an anti-Bush gesture rather than a show of support.

Thirty-one percent of respondents said the unpopular war in Iraq was uppermost in their minds, as opposed to 21 percent for the economy, 12 percent health care and 11 per cent the fight against terrorism, Bush's strong point.

Control of Congress would provide the Democrats with a powerful tool for controlling the political agenda over the next two years and launching a series of public investigations into Iraq and other Bush policies.

But the White House has strongly signalled that it will not change course in Iraq, with Vice President Dick Cheney saying in a recent interview that "it doesn't matter" that most of the US public has turned against the war.

Washington pundit Charlie Cook forecast "a very strong Democratic wave" on election day. Analysts gave the party a strong shot to win the House but said the Senate hinged on a few key races in states such as Virginia and Missouri.

Voters in 36 states also are casting votes for governor Tuesday. Democrats hold significant leads in five of nine open seats: Ohio, Arkansas, Colorado, New York and Massachusetts, and are favoured to displace Republicans in Maryland, Arkansas, and Minnesota.

That sets up the possibility that Democrats could gain a solid majority of governorships for the first time since 1994.