Tammy Baldwin is entering the race for Wisconsin's U.S. Senate seat being vacated by retiring Democrat Herb Kohl.
Baldwin made her announcement Tuesday in an email and video message to supporters, saying she would fight for middle-class voters.
Baldwin, of Madison, is the first Democrat to officially get into the race as her party looks to maintain its Senate majority.
If elected, Baldwin would become the first openly gay member of the Senate. She was first elected to Congress in 1998.
Former Republican Congressman Mark Neumann entered the race last week. Several other Republicans, including former Gov. Tommy Thompson and Wisconsin Assembly Speaker Jeff Fitzgerald, are also expected to run.
On the Democratic side, Congressman Ron Kind of La Crosse is among those considering a bid.