Besides casting ballots for president, U.S. voters are also electing members of Congress.
All 435 seats in the House of Representatives are up for election, as are 34 of the 100 seats in the Senate.
The House seats are for two-year terms, and those in the Senate are for six-year terms.
Analysts predict Republicans will maintain their narrow lead in both chambers.
Democrats hope they might wrest control in the Senate, where Republicans have a two-seat majority.
But that may be difficult, as a large number of Democratic incumbents are retiring from Republican-leaning Southern states. They include Senators John Breaux of Louisiana, Bob Graham of Florida, Fritz Hollings of South Carolina, and John Edwards of North Carolina, Democratic Presidential candidate John Kerry's running mate.
Republican-controlled Senate seats are at risk in four states: Alaska, Colorado, Oklahoma and Kentucky.
The Senate contest that has received the most attention is South Dakota, where Democratic Minority Leader Tom Daschle is in a close race with his Republican challenger, former Congressman John Thune in a state that voted for President Bush by a 22-point margin over Al Gore four years ago.
In another close race, Republican Senator Lisa Murkowski of Alaska is facing a tough challenge from former Democratic Governor Tony Knowles. Senator Murkowski was appointed to her position by her father in 2002 when he resigned the Senate seat to become governor, and the issue of nepotism has dogged her campaign.