A Republican source has told RealClearPolitics that former Wisconsin Gov. Tommy Thompson will not challenge Sen. Russ Feingold (D).

A Milwaukee radio station first reported that Thompson is scheduled to announce his plans tomorrow at a Tea Party rally in Madison. While the AP now reports that Thompson hasn't made up his mind, he's still not expected to run.

The news is a major hit for Republicans. According to recent polls, Thompson represented the party's best chance to defeat the three-term senator. Feingold currently leads other potential opponents Terrence Wall and Dave Westlake by more than 10 points in the RCP Average, though that will likely shrink as Feingold has never been elected with more than 55 percent of the vote.

The latest poll, released earlier this month, found Thompson up by 12 points. He leads Feingold by 3.8 points in the RCP Average.

This is the second piece of bad news for Republicans in the last day. Former New York Gov. George Pataki (R) announced yesterday that he will not challenge Sen. Kirsten Gillibrand (D-N.Y.), despite regularly leading her in polling.

The Thompson news is arguably a greater loss, however, since national Republicans never expected Pataki to run. It also makes the path to majority control of the Senate far narrower.