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."