Republicans and Democrats have sent thousands of volunteers to the most contested US states to canvass support ahead of Tuesday's mid-term elections.
Hundreds of thousands of voters have cast early ballots
President George W Bush has been rallying support in the southern states of Florida, Texas and Arkansas.
Democrats, whose lead has narrowed in final 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.
Addressing Republicans in Florida on Monday, President Bush said: "I knew we were going to finish strong. Republicans are going to turn out and it's going to be a great victory."
The BBC's Justin Webb in Washington says the president is making a late but vigorous effort to persuade Americans that continued Republican control of Congress is vital, and it does seem likely that the party will keep hold of the Senate at least.
Recent opinion polls suggest the Democrats will pick up seats, and that the Iraq war and the president himself are unpopular.
But the final polls also suggest a slight gain in Republican fortunes and that gain could result in the president's party rescuing its fortunes at the last minute, our correspondent 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.
'A world better off'
The war in Iraq has dominated much of the campaign. Both parties welcomed the death sentence handed down to former Iraqi leader Saddam Hussein on Sunday.
The Republicans say if the Democrats win control of Congress they will pull out of Iraq prematurely, while the Democrats have attacked what they say are Mr Bush's failed policies in Iraq.
"My decision to remove Saddam Hussein was the right decision, and the world is better off for it," Mr Bush said to crowds of supporters in Florida on Monday.
"The only way we will not win is if we leave before the job is done. If you listen for the plan from the Democrats, there isn't one," he said.
However, US policy in Iraq has been heavily criticised in an editorial published in an influential military journal.
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".
It urged President George W Bush to install a replacement at the Pentagon.
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 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.