U.S. Rep. Tammy Baldwin of Wisconsin entered the race Tuesday for the U.S. Senate seat being vacated by retiring Democrat Herb Kohl, becoming the first Democrat to officially jump in to what will be an important race for her party to win in order to maintain majority control.

One of the most liberal members of Congress, Baldwin had been saying since Kohl announced his retirement in May that she was seriously considering a Senate bid. Her congressional district includes the city of Madison, a liberal Democratic stronghold, and some surrounding rural areas.

Baldwin, 49, made her announcement in an email and video announcement to supporters early Tuesday. If elected, she would become the first openly gay member of the Senate.

Baldwin, the first woman who Wisconsin voters sent to Congress, was also the first person elected to Congress after announcing they were gay. She was first elected in 1998.

The Wisconsin seat is one of at least eight open spots that will help determine the balance of power in the Senate, where Republicans need to pick up just four seats to take control.

In her message to supporters, Baldwin tried to move herself to the political center by positioning herself as the best candidate for middle-class voters feeling squeezed in the tough economy.

"It's time politicians looked out for seniors, working families and the middle class _ instead of protecting the profits of big oil and Wall Street," she said.

Baldwin also used her video message to mention her opposition to the war in Iraq and her support for ending the war in Afghanistan, as well as to hint at the obstacles her candidacy will face as she seeks to win her first statewide election.

"I'm used to facing challenges head on," she said. "When I first ran for Congress in 1998, people counted me out. But we worked hard, campaigned across south-central Wisconsin, and we won."

Republicans are sure to go after Baldwin's liberal voting record, hoping to sway independent and moderate voters their way in a state that has swung between handing President Barack Obama a 14-point win in 2008 and kicking Democrats out of power in the Statehouse in 2010.

Former Republican U.S. Rep. Mark Neumann entered the Senate race last week. Neumann said at the time that he was focusing his campaign on Baldwin, who he presumed would capture the Democratic nomination.

But there promises to be a spirited contest on the GOP side, with longtime Gov. Tommy Thompson making serious moves toward his first run for office since 1998. Other Republicans indicating they plan to run include Assembly Speaker Jeff Fitzgerald, a top ally of polarizing Gov. Scott Walker; state Sen. Frank Lasee, a lawmaker who once advocated arming teachers to protect their classrooms; and former state Sen. Ted Kanavas, a lower-profile candidate who's been quietly building support.

On the Democratic side, U.S. Rep. Ron Kind of La Crosse is considering running, as is former two-term U.S. Rep. Steve Kagen of Appleton. Former Sen. Russ Feingold, who lost re-election last year to Republican Ron Johnson, has said he wouldn't run for any office in 2012.