If Russ Feingold thought he’d escaped a tough re-election bid with the demurral of Tommy Thompson last month, the latest Rasmussen numbers come as an unpleasant reminder of his tenuous grip on office.  Wisconsin Republicans endorsed businessman and newcomer Ron Johnson in last week’s state convention as the nominee to challenge Feingold, and he’s already caught the incumbent in a virtual tie among likely voters:

Businessman Ron Johnson, endorsed at last weekend’s state Republican Convention, is now running virtually even against incumbent Democrat Russ Feingold in Wisconsin’s race for the U.S. Senate.

A new Rasmussen Reports telephone survey of Likely Voters in Wisconsin shows Feingold with 46% support to Johnson’s 44%. Three percent (3%) prefer some other candidate, and six percent (6%) remain undecided.

As he has in surveys since the beginning of the year, Feingold continues to fall just short of 50% regardless of which Republican he’s matched against. Incumbents who earn less than 50% of the vote at this stage of a campaign are considered potentially vulnerable. Feingold was reelected to a third term in 2004 with 56% of the vote.

It’s a tight race even in the internals.  Surprisingly, Johnson wins a two-point edge among 18-29YO voters, but apart from an eleven-point lead among 40-49YO voters, the age demographics are all narrowly split.  The income demographics show more definition between the candidates, but Johnson wins more of them than does Feingold.  Feingold takes a six-point lead among independents, 44-38,  but 10% are still undecided.

It won’t help that Wisconsin voters favor repealing the ObamaCare bill 53/38, and favor passage of an Arizona-like immigration-enforcement law in their state, 57/29.  Unlike other Democratic incumbents in blue states, Barack Obama is just underwater in approval, 49/50.  The current Democratic Governor is farther underwater at 41/57.

Feingold does have a decent favorability rating at +9, 53/44, but with only 3% having no opinion.  He’s well defined, thanks to his long incumbency.  Johnson, on the other hand, has 32% of the likely voters in Wisconsin to convince one way or the other.  Among the rest, he has a +17 at 42/25.  If Johnson can define himself in this race before Feingold can do the job for him, he has plenty of room to improve his standing and his numbers.  For Feingold, it looks as though he’s near his ceiling already.