Congressional elections are held every two years in the United States.
There are 33 senate seats in contention on election day 2012. Senators serve a six-year term.
All of the seats in the House of Representatives are open. Representatives, also called "Congressmen" or "Congresswomen," are up for re-election every two years.
Senate
Voters are electing people to fill 33 seats, about one-third of the Senate. Republicans need to gain four seats to take control from the Democrats.
Here are some of the key races:
Democratic Rep. Joe Donnelly vs. Republican Richard Mourdock
Republicans initially expected a Mourdock victory but pre-election polls indicate the candidate's controversial remarks about rape and abortion have put a dent in his lead over Donnelly.
Massachusetts
Democrat Elizabeth Warren vs. Republican Senator Scott Brown
Brown has attempted to appeal to a wide range of voters, even featuring President Obama in campaign ads in his race against Warren, a consumer rights advocate.
Missouri
Democratic Sen. Claire McCaskill vs. Republican Rep. Todd Akin
Analysts initially considered McCaskill vulnerable, but comments from Akin about rape and pregnancy drew strong criticism from fellow Republicans, with some asking him to pull out of the race.
Virginia
Democrat Tim Kaine vs. Republican George Allen
The two candidates, both former Virginia governors, are in a tight race in a state which could prove pivotal in determining the next president.
House of Representatives
All 435 seats in the House of Representatives are up for grabs. Republicans currently hold a majority in the House. Democrats need to gain 25 seats to take control.
Here are some of the key races:
Florida
Democrat Patrick Murphy vs. Republican Rep. Allen West
West may have alienated some of his supporters earlier this year when he referred to some Democrats in Congress as Communist Party members.
Illinois
Democrat Tammy Duckworth vs. Republican Rep. Joe Walsh
Duckworth, an Iraq War veteran who lost both legs in a helicopter crash, and Walsh have sparred over issues ranging from Medicare to abortion in their tight race.
Minnesota
Democrat Jim Graves vs. Republican Rep. Michele Bachmann
Bachmann, who sought her party's nomination for president, is now seeking a fourth term in the House in her race against Graves, a hotel executive.
Utah
Democratic Rep. Jim Matheson vs. Republican Mia Love
Six-term incumbent Matheson is in a tight race against Love, an up-and-coming Republican, the Mormon daughter of Haitian immigrants who has received strong party support.