If Republicans have taken to calling Scott Brown ’41’ as a nickname after his victory in Massachusetts, perhaps Democrats can start calling Russ Feingold ’43.’  No, that’s not to say that Feingold will switch parties and bring a friend along for company.  That’s the percentage of likely voters in Wisconsin that would support Feingold over potential challenger and former Governor Tommy Thompson in a head-to-head match this fall:

One more Democratic senator who has long been regarded as a safe prospect for reelection may be facing a challenging year in 2010.

A new Rasmussen Reports telephone survey of likely voters in Wisconsin finds Republican Tommy Thompson edging incumbent Russ Feingold 47% to 43% in a hypothetical U.S. Senate match-up. Five percent (5%) like some other candidate, and four percent (4%) are undecided.

Any incumbent who attracts less than 50% support at this point in a campaign is considered potentially vulnerable.

Thompson, who served as governor of the state from 1987 to 2001 and as secretary of Health and Human Services in President George W. Bush’s first term, is being urged by Republicans to enter the race. However, it remains unclear if he will enter the race. Feingold is seeking a fourth six-year term in the Senate this November.

Until just recently, people calculating risk for Democratic incumbents have mostly focused on the obvious problems in otherwise red states.  Arkansas (Lincoln) and Indiana (Bayh) have gotten a lot of attention, as has Nevada, which is more independent than red but where Majority Leader Harry Reid is drowning in voter anger over the Obama-Pelosi agenda.  Massachusetts made it clear that no state was entirely safe for Democrats, but in Massachusetts, Democrats didn’t have a longtime incumbent running in the race, either.

Wisconsin could be a big surprise, too.  It has sent reliable liberals like Feingold and Herb Kohl to the Senate for several cycles (21 years for Kohl, 17 years for Feingold), but outside of the big college towns, the state is more conservo-populist, not unlike the Dakotas.  Feingold has built a reputation for straight talk which has kept his constituents’ respect even when Feingold goes more to the left than they do.

Those days are apparently over.  Not only does Feingold trail Thompson by four points, his job approval numbers have gone underwater, 47/48.  The voting public has also turned more substantially away from Feingold on policy.  Fifty-nine percent want to see tax cuts as a cure for a bad economy, against only 15% for more government spending.  Almost two thirds (65%) reject the Democrats’ argument that the economy is improving (41% say worse, 24% says it’s the same), while only 28% believe it’s improving.

Can Barack Obama help Feingold in Wisconsin?  Obama won by a much larger margin than John Kerry did in 2004, but he’s not winning any more.  His job approval among likely Wisconsin voters has dropped underwater, with a majority disapproving (54%/46%, no one unsure).  Furthermore, the Democratic Governor, Jim Doyle, has even worse approval numbers — 36/62. He won’t be any help to Feingold, and may well help stoke Republican and independent turnout in the fall.

Thompson has not yet committed to this race, but the seat may be his for the asking.  It looks like 2010 will be at least as bad for Democrats in Wisconsin as anywhere else, and perhaps worse.

Update: Kevin at Lakeshore Laments says that Thompson needs to stop his “Hamlet-Brett Favre act” to be taken seriously.