Voting was scheduled to begin throughout the country early Tuesday. Bush seized on the death sentence handed down to former Iraqi President Saddam Hussein on Sunday to reinforce the case for the war in Iraq that nearly 60 percent of Americans now say is not worth fighting for.
"Today, we witnessed a landmark event in the history of Iraq: Saddam Hussein was convicted and sentenced to death by the Iraqi high tribunal," Bush told cheering supporters at a rowdy Republican rally in Nebraska.
"It's a major achievement for this young democracy," he said, urging the crowd to "give our thanks to the men and women of America's armed forces. Without their courage and skill, today's verdict never would have happened."
One day before Tuesday's vote, dozens of congressional races were too close to call and two national public opinion surveys favoured Democrats overall but suggested a late burst of energy among Bush's Republicans.
Bush was in Nebraska and Kansas Sunday as part of an 11th-hour political rescue mission for struggling Republican candidates facing unexpectedly strong Democratic challenges in part because of anger at the war in Iraq.
"Saddam Hussein was a threat," he said, amid signs the conflict could cost his party control of the US Congress. "My decision to remove Saddam Hussein was the right decision, and the world is better off for it."
White House spokesman Tony Snow dismissed as "preposterous" any suggestion that US and Iraqi officials manipulated the timing of the sentence to help the Republicans 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, where a raging insurgency and escalating sectarian strife have 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 need a net gain of 15 seats out of the total 435 in play on Tuesday to control the House. They hope 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 poll 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 Center 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.
Meanwhile, a new USA TodayGallup survey published on Monday showed Democratic challengers were likely to win US Senate races in Missouri, Montana, Rhode Island, Ohio and Pennsylvania. That meant that toss-up contests in Tennessee and Virginia will determine if Democrats gain control of the upper chamber.
Control of Congress would provide the Democrats with a powerful tool for controlling the political agenda over the next two years, allowing them to launch 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 hinges on a few key races in states such as Virginia and Missouri.
Voters in 36 states also are casting votes for governors 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.

Copyright AFP (Agence France-Presse), 2006