President Bush cast his ballot early Tuesday in Crawford, Texas, where he urged people across the country to get out and vote.
"Our government is only as good as the willingness of our people to participate, and therefore, no matter what your party affiliation or if you do not have a party affiliation, do your duty," he said. "Cast your ballot, and let your voice be heard."
All 435 seats in the U.S. House of Representatives are up for grabs, as are 33 seats in the 100-member Senate. Voters will also be choosing governors in 36 states, and deciding a variety of referendum issues, from gay marriage to embryonic stem cell research.
Democrats are hoping that dissatisfaction with the war in Iraq will help them retake control of one or both houses of Congress.
"People in this country really want a new direction, and I think you are going to see a big turnout, and I think we are going to get a new direction in the country," said Howard Dean, chairman of the Democratic Party.
Public opinion polls taken in the run-up to the election indicate Democrats have a good chance of recapturing a majority in the House, and making gains in the Senate. Democrats need to win an additional 15 seats in the House and six seats in the Senate to take control of both chambers.
Among the closest races are those for the Senate in Missouri, Virginia, Tennessee, Montana, and Rhode Island, all Republican-held seats.