Wisconsin Rep. Tammy Baldwin on Tuesday declared her candidacy for the U.S. Senate seat being vacated by retiring Senator Herb Kohl, becoming the first openly gay Senate candidate with a viable shot at winning a seat.

After Kohl announced in May he would not seek reelection in 2012, Baldwin said she was considering a bid for the Democratic nomination. The Victory Fund, a group dedicated to electing openly gay candidates, urged Baldwin to enter the race and promised to make her candidacy a high priority.

The 49-year-old Baldwin made her announcement in an email to supporters that included a videotaped message. She is the first Democrat to enter the field.

“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 in her message.

“I'm used to facing challenges head on,” she added. “When I first ran for Congress in 1998, people counted me out. But we worked hard, campaigned across south-central Wisconsin, and we won.” (The video is embedded in the right panel of this page.)

Baldwin is among the four openly gay lawmakers serving in the House. Also gay are David Cicilline of Rhode Island, Barney Frank of Massachusetts and Jared Polis of Colorado. Before being elected to Congress, she served 8 years in the Wisconsin Assembly.

Baldwin is a perennial favorite among her constituents – she won 63% percent of the vote on November 2 – but remains untested in a statewide election.

Former Wisconsin Rep. Mark Neumann has announced he'll seek the GOP nomination. The two-term former congressman in 1996 said, “If I were elected God for a day, homosexuality wouldn't be permitted.”