Candidates have made their final push for votes, with the US Congressional mid-term elections due to start within hours.
President Barack Obama's Democratic party is expected to lose its majority in the House of Representatives and is struggling to keep the Senate.
Mr Obama spent Monday at the White House, while First Lady Michelle Obama campaigned in Nevada and Pennsylvania.
Republicans hope to capitalise on voters' discontent with the economy.
"We're hoping now for a fresh start with the American people," Republican National Committee chairman Michael Steele said.
"If we don't live up to those expectations, then we'll have a problem in two years."
Up for election on Tuesday are all 435 House seats, 37 of the 100 seats in the Senate, governorships of 37 of the 50 states and all but four state legislatures.
The Republican Party needs to gain 39 House seats to win control of the lower chamber of Congress - an amount opinion polls suggest they will easily win - and 10 to take over the Senate.
Democrats are hoping to hold on to the Senate by at least one or two seats.
In addition, voters will decide on ballot measures ranging from marijuana legalisation in California to a referendum in Oklahoma on forbidding judges from using Islamic Sharia law in rulings.
Mr Obama tried to rally support for Democrats on Monday by giving an interview to Ryan Seacrest, host of the popular TV programme American Idol. The interview is due to air on Tuesday on Seacrest's nationally syndicated radio show.
Mr Obama also recorded interviews for radio stations in the cities of Milwaukee, Cincinnati, Philadelphia, Honolulu and Miami.
Meanwhile, Michelle Obama was in Nevada to support the campaign of Harry Reid, the Democrats' Senate majority leader, who is facing a knife-edge battle to keep his seat.
"Can we do this? Yes, we can. Yes, we must," she told the crowd, updating the slogan that carried her husband to the White House two years ago.
Former President Bill Clinton rallied Democrats a day before the elections in the states of New York, Pennsylvania and West Virginia.
Mr Clinton told an enthusiastic crowd in the Pennsylvania town of McKeesport that the most important thing was to get out the vote on Tuesday.
Both Republicans and Democrats appeared on early morning TV shows to try to motivate supporters for election day.
Democratic National Committee chairman Tim Kaine and Mr Steele appeared on the ABC network's Good Morning America, Democratic Senator Dick Durbin of Illinois and Republican Senator John Cornyn of Texas were interviewed on NBC's Today show and Mississippi Governor Haley Barbour was on CBS's The Early Show.
Mr Steele acknowledged that recapturing the Senate would be a difficult task, while Mr Barbour said claiming a majority in the Senate would be "a bit of a stretch".
Tea Party momentum
Republicans are riding high in opinion polls, buoyed by discontent over unemployment running near 10%, slow economic growth and a burst housing market bubble that has seen many Americans lose their homes.
The party has also gained from the backing of the populist Tea Party movement, which has given voice to conservative grassroots opposition to Mr Obama and the Democrats' economic stimulus programmes and healthcare overhaul.
Tea Party favourite Sarah Palin, the Republican vice-presidential candidate in 2008 and a former governor of Alaska, said in a TV appearance on Sunday that Tuesday's vote would be a "political earthquake".
She said voters would tell the president and the Democrats: "You blew it, President Obama. We gave you the two years to fulfil your promise of making sure that our economy starts roaring back to life again."
Ms Palin is not running for any office but has hinted at a 2012 run for the presidency.
Republican House leader John Boehner, who stands to replace Democrat Nancy Pelosi as the House Speaker if the polls are borne out, acknowledged the country's economic problems had not started under Mr Obama.
"But instead of fixing them, his policies have made them worse," he said, campaigning in Ohio.
The results of the day's gubernatorial and state legislative elections could also have a big impact on American politics.
In a given state, the party that controls the state legislature and holds the governor's office has influence over the redrawing of the Congressional district map for the next 10 years.