Weigel

Republicans probably should have won control of the Senate during their 2010 midterm sweep. But time and again, Tea Party activists chose the more conservative (and polarizing) candidate in GOP primaries, making it easy for Democrats to portray them as extremists.

We saw this with Harry Reid, the Democratic Majority Leader, in Nevada, where despite being extremely unpopular, he was able to hang on because his opponent, Sharron Angle, had suggested she would scrap Social Security and force rape victims to bear their children. Ditto in Delaware, where Tea Party favorite Christine O'Donnell took out Rep. Mike Castle in the primary, only to get wiped out in November.

Indiana may prove to be this year's showcase for ideological furor getting in the way of Republican Senate dreams. Here's the latest ad from Hoosier Democrats on behalf of their candidate, Rep. Joe Donnelly, excoriating Tea Party-backed state Tresaurer Richard Mourdock (who took out incumbent moderate Senator Richard Lugar in the primary):

Barack Obama won Indiana in 2008, but just barely, and is not making a serious play for the state this time around. But if Donnelly can tap into the president's support while retaining a distinct identity among moderate white voters, he's got a real shot.