Wisconsin Congresswoman Tammy Baldwin has announced she will run for the U.S. Senate, and undeclared Elizabeth Warren leads Democratic contenders in Massachusetts. (More)

The 2012 Senate races will pose a major challenge for Democrats. Democrats have 23 seats in play and six of those open after retirements, while Republicans have only 10 seats in play with two open. The simple math puts Democratic control of the Senate in jeopardy, and Sean Trende at Real Clear Politics sees the challenge as even steeper. Trende rates only John Ensign (NV) and Scott Brown (MA) as “vulnerable” among GOP incumbents. The GOP retirements are in Texas and Arizona, both won by Republican presidential nominee John McCain in 2008. In contrast, Trende rates seven Democratic seats as “vulnerable,” including the North Dakota seat to be vacated by Kent Conrad, and five more as “potentially vulnerable” including the Wisconsin seat left open by Herb Kohl.

In Massachusetts, Elizabeth Warren has not yet declared her candidacy but a WBUR poll this week shows her leading other Democratic contenders and trailing incumbent Scott Brown by only 9 points. More important, almost 44% of the voters polled had never heard of Warren. That’s good news in that she has plenty of room to grow. It’s bad news in that an announcement of her candidacy would set off a race to establish her image. She would need to spend a lot of money, quickly, before Sen. Brown and Warren’s Democratic opponents paint their own images of her.

In Wisconsin, Rep. Tammy Baldwin announced her candidacy yesterday. She appears the favorite for the Democratic nomination, especially after former Senator Russ Feingold announced two weeks ago that he would not run for any office in 2012. The first openly-lesbian non-incumbent to win a U.S. House seat, Rep. Baldwin has won landslides in her Dane County district. But she has not yet faced a statewide race, and her Republican opponent will almost certainly make her sexual orientation an issue with religious voters. The leading Republican contenders so far are Club for Growth-backed Mark Neumann, former governor Tommy Thompson, and Wisconsin Assembly Speaker Jeff Fitzgerald.

With 23 Senate seats in play, 12 rated at least “potentially vulnerable,” and only a three-seat majority after 2010, Democrats have our work cut out for us next year … and we have no margin for error.