With 33 Senate seats up for grabs in 2012, Democrats will be defending 21 (plus two independents who caucus with them) in a fight to hold on to their majority. And 25 incumbents in all will be up for election. Here's a look at some of the 2012 Senate contenders who have already thrown their hat into the ring:
Nevada: Rep. Shelley Berkley (D)
After months of speculation, Berkley said on April 14 that she will run for the seat being vacated by retiring Sen. John Ensign (R). If she makes it through the Democratic primary, she'll likely enter a general election showdown with Republican Rep. Dean Heller. It would be the first Senate race this cycle where two sitting House members are running against each other.
"The problem in Washington is that Republicans like Dean Heller are clinging to extreme ideology rather than working on solutions that address the challenges facing our state," Berkley said in a statement. "As I talk to Nevadans across our state I hear one priority: jobs, jobs, jobs."
Hawaii: Former Rep. Ed Case (D)
Case announced on April 10, in an e-mail to supporters and a video posted online, that he plans to run for retiring Sen. Daniel Akaka's seat. "I'm running for the Senate because I believe we can and must do better, and that our Hawai'i offers a better way forward for our country," Case said in his e-mail.
Case isn't likely to be the only well-known Democrat entering Hawaii's race. Other possible candidates include current Reps. Colleen Hanabusa and Mazie Hirono, and former Honolulu mayor Mufi Hannemann. Anyone who does run against Case in the Democratic primary is likely to run to his left; he compiled a largely centrist record while in Congress, creating some enemies among the state's liberal Democratic base.
Virginia: DNC Chairman Tim Kaine
Kaine announced on April 5 that he will enter Virginia's Senate race. His decision provides Democrats with an elite contender in a competitive race for an open seat the party currently holds.
In a video announcing his decision Kaine, a former governor, said, "I'm running for the United States Senate because America has big challenges, and I'm convinced Virginia has answers to help strengthen our nation."
Virginia: Former Sen. George Allen (R)
Allen announced on January 24 that he would run for his former Senate seat. Allen's decision was no surprise; he had spent months shoring up support within the Virginia GOP.
"Today, I'm announcing my candidacy for the U.S. Senate," Allen said in his announcement video. "You know me as someone willing to fight for the people of Virginia. And I'd like the responsibility to fight for you again."
Allen's reelection bid -- as well as his career -- was derailed in 2006 when a video caught him derisively calling a Democratic opposition researcher "macaca" in a campaign appearance. Up to that point, Allen was leading in the polls and expected to win reelection. He was also being floated as a 2008 presidential contender.
Arizona: Rep. Jeff Flake (R)
The sixth-term congressman made his Senate bid official on February 14, following Republican incumbent Jon Kyl's retirement announcement the previous week.
Though he has no declared opponents so far, Flake's campaign is already under way. His campaign reported raising over $1 million in the first three months of 2011.
Florida: Former Sen. George LeMieux (R)
LeMieux officially announced his Senate candidacy on April 5, saying that he is running because he believes "the stakes have never been higher."
LeMieux joins state Senate President Mike Haridopolos and former state House Majority Leader Adam Hasner in the GOP race. The winner of the primary will face Sen. Bill Nelson (D), who is running for his third term.
LeMieux was appointed to the Senate in 2009 by Gov. Charlie Crist when Sen. Mel Martinez (R) left the Senate before his term was up. Before that, LeMieux was Crist's chief of staff. LeMieux recently defended his past relationship with Crist, who left the GOP to run for Senate as an independent last year, saying, "When he walked away from the party and our policies, I didn't walk with him."
Connecticut: Rep. Chris Murphy (D)
The third-term congressman announced on January 20 that he was running for the seat of retiring Sen. Joe Lieberman (ID).
"I only take this step because I believe I can be an even stronger voice for change in the United States Senate," Murphy said in a statement. Murphy also made a video announcement on his website and hosted an event the evening of his announcement in Cheshire, Conn.
Murphy reported raising $1.1 million in the first quarter of 2011, more than double that of his Democratic primary opponent, former Connecticut Secretary of State Susan Bysiewicz.