It's not just a president who gets elected on Tuesday. Americans will also cast ballots for hundreds of legislators in the two chambers of Congress, the Senate and the House of Representatives, as well as 13 governors.
The US Constitution grants all legislative powers to Congress. Any legislation must be passed in identical form by both the House and Senate before it is signed into law by the president.
HOUSE OF REPRESENTATIVES
Speaker of the House: John Boehner, Republican
Minority leader: Nancy Pelosi, Democrat
All 435 seats in the House are up for election every two years. Each state has a number of congressional representatives, apportioned by population, a figure that is updated every 10 years based on results of the US census.
Republicans hold 240 seats, the Democrats 190, and five seats are vacant.
Because of their two-year terms, representatives are in constant campaign mode, and the two parties have raised a combined $977 million during this election cycle.
The class of 2010 saw a record 100 "freshmen" sweep into the House, largely on a Republican wave.
In total there are 76 women members, 30 Hispanic-Americans, 43 African-Americans and nine Asian-Americans.
Boehner seems almost certain to keep his job, as Democrats would need to gain at least 25 seats to take control.
Law dictates that all revenue-raising legislation originate in the House, but like other bills it must also pass the Senate.
The sole power of impeachment of senior officials, including the president, lies with the House, while the Senate tries and votes on such impeachment.
In the case of a tie in the state-by-state electoral college in the presidential election, the vote goes to the House, where delegations from each of the 50 states cast the deciding votes.
Majority Leader: Harry Reid, Democrat
Minority Leader: Mitch McConnell, Republican
The Senate's 100 members serve six-year terms, and one third of the seats are up for election every two years. Each state, regardless of size, has two senators. Tiny Delaware, with less than one million people, has the same representation in the Senate as California, with its 38 million residents.
Democrats currently hold 51 seats, plus two independents who caucus with the party, while Republicans hold 47.
By Constitutional authority, the vice president serves as president of the Senate, but can only vote in order to break a tie.
While the House more directly reflects the "will of the people" through district representation, senators represent their entire state.
House bills often move quickly, while the Senate is slower and deliberative. Legislation gets poured "into the senatorial saucer to cool it," said George Washington, the nation's first president.
The Senate has been historically less partisan than the House, but the lines have hardened in recent years. No Republican senator, for instance, voted for the president's health care reform bill in late 2009, and McConnell stated in 2010 that Republicans' "top political priority" should be to prevent Obama's re-election.
The Senate is responsible for confirming the president's Supreme Court nominees, as well as candidates to be federal judges, US ambassadors, and for other key posts.
Seventeen women and two Hispanics serve in the Senate; there are currently no African-American Senators.
Also on November 6, 11 US states and two US territories will elect governors. Nine of the governorships up for grabs are currently held by Democrats and four by Republicans.