President George W Bush is rallying support in the southern states of Florida, Texas and Arkansas on Monday.
Democrats, whose lead has narrowed in recent opinion polls, are focusing on Iraq, saying Republicans have blindly followed Bush's "failed policy".
Democrats hope to win control of at least one of the Houses of Congress.
The BBC's James Coomarasamy in Washington says that although the mid-terms are essentially a series of local elections, the president's low approval ratings and the high number of recent US casualties in Iraq have made many races less predictable.
But the Republicans have been campaigning hard and may have had a boost in recent days, he adds.
Hundreds of thousands of voters have already cast their ballots, taking advantage of an early voting system.
Thirty-five states and the District of Columbia allow in-person voting before election day in certain cases - either at a voting machine or by absentee ballot.
However, turnout on Tuesday is not expected to be above 40%, and both parties are spending the last day trying to ensure their supporters are motivated to vote.
'Courage and skill'
Iraq has dominated the campaign season. Both parties welcomed the death sentence handed down to former Iraqi leader Saddam Hussein on Sunday.
However, US policy in Iraq has been heavily criticised in an editorial published in an influential military journal on Monday.
The Army Times, Air Force Times, Navy Times and Marine Corps Times said Defence Secretary Donald Rumsfeld, the architect of the Iraq campaign, had "lost credibility with the uniformed leadership".
Mr Bush says Saddam Hussein's sentence is a landmark
Mr Bush is canvassing support in three southern states on Monday, a fifth successive day on the campaign trail in areas judged vulnerable by Republican pollsters.
On Sunday he spoke in Nebraska, saying the verdict of execution against Saddam Hussein was a "landmark event".
He urged his audience to thank "the men and women of America's armed forces" for their courage and skill, without which the "verdict would never have happened".
The Republicans have regularly accused the Democrats of cowardice over Iraq, saying the party is not prepared to take the risks necessary to make the US secure.
The Democrats say they want a "new direction" to Iraq.
Correspondents say the Democratic party could push to investigate the Bush administration preparations for the Iraq war and there are also those within the party who want to impeach the president for allegedly misleading Congress about Iraq's weapons programmes.
A third of the Senate, the whole House of Representatives and 36 governorships are up for election on 7 November.
The Democrats need to pick up six seats to gain control of the Senate, and 15 House seats to have a majority there.
Source: BBCGüncelleme Tarihi: 20 Eylül 2018, 18:16