Rep. Tammy BaldwinTammy BaldwinSchumer: 'Very good chance' Price won't be confirmed Trump, House GOP could clash over 'Buy America' WHIP LIST: How many Dems will back Sessions? MORE (D-Wis.) announced Tuesday that she'll run for Senate in 2012, making her candidacy official in a Web video.

Baldwin, who's been exploring running to succeed the retiring Sen. Herb Kohl (D), positioned herself as a "progressive fighter for Wisconsin," and a candidate who would govern in the mold of Kohl and former Sen. Russ Feingold (D-Wis.).

"I've heard story after story from people across Wisconsin about how their families and communities have been hurt by the economy. And they feel like no one cares; like no one's listening to them," Baldwin says in the video. "That's why I'm running for the U.S. Senate — to stand up for you, no matter how tough the odds, or how powerful the special interests it means fighting against."

She enters the race as the early front-runner for the Democratic nomination. Feingold passed on an opportunity to regain a seat in the Senate, and liberal activists have been enthusiastically supportive of Baldwin's candidacy since Kohl announced his retirement.

On the Republican side, former Gov. Tommy Thompson (R) has been exploring running for the seat, though conservative activists have expressed their doubts about his bona fides. Former Rep. Mark Neumann (R-Wis.) is also running for the seat.