Several Democratic House members are eyeing the seat while GOP hopes to make it competitive.
The controversial 2012 GOP Senate nominee now says he won't challenge Sen. Roy Blunt (R-Mo.).
The Maryland Democrat is the longest-serving woman in congressional history.
Ashley Swearengin will not run to replace retiring senator.
An ad targeting Gov. Maggie Hassan (D) is running ahead of her potential Senate bid.
Controversial Missouri Republican says he's considering a primary against Sen. Roy Blunt.
The announcement sets up a fight between two of the most prominent Ohio politicians.
Former Los Angeles mayor's decision makes Kamala Harris (D) a heavy favorite for open seat.
Former California Republican Party Chairman Tom Del Becarro has formed an exploratory committee for the seat, while Republicans continue to push former Rep. David Dreier (R-Calif.) to jump into the race as well.
The former Democratic governor will reportedly challenge Sen. Rob Portman (R) in 2016.