Republicans meanwhile have a number of incumbents likely to seek reelection in Democratic-leaning states, meaning control of the chamber could well flip again in 2016.
Here is an early look at what the Senate battleground map looks like.
Nevada: Republican Gov.
California: Sen. Barbara Boxer was the first Democrat to announce retirement plans. But the early action has been all about which Democrat will replace her, given the dearth of viable options on the Republican side. Atty. Gen. Kamala Harris is seen as the early favorite, but Southern California Democrats, including a number of House members, are considering jumping in as well.
Maryland: The retirement of Sen. Barbara Mikulski, the longest-serving woman in the history of
New Hampshire: First-term Sen.
Illinois: Mark Kirk claimed what had been Barack Obama's Senate seat in 2010, and Democrats are eager to take it back. Kirk has said he will run for reelection despite his ongoing recovery from a massive stroke in early 2012. Rep.
Pennsylvania: In an increasingly Democratic-tilted state, Sen.
Ohio: Like Florida, Ohio could be a battleground for president and Senate.