Thompson has not announced he would run for the seat, although he told the Milwaukee Journal-Sentinel last week that he was considering bids for the Senate seat or the governor's job now that Democrat Jim Doyle has decided not to seek a third term. Thompson even said he might run for mayor of his hometown of Elroy.

Feingold, seeking a fourth term, has bigger leads over two lesser-known Republicans who have announced: real estate developer Terrence Wall and businessman David Westlake.

Feingold leads Wall 48 percent to 34 percent with 19 percent undecided, and Westlake by 47 percent to 32 percent with 21 percent undecided.

Forty-five percent of voters approve of Feingold's job performance compared to 37 percent who do not, with 18 percent undecided.

Forty-five percent have an unfavorable view of Thompson compared to 38 percent who see him positively and 17 percent undecided.

Eighty-five percent of voters don't know enough about Wall to express an opinion and the same is true of 88 percent in Westlake's case.

"A lot of Democrats were scared yet another of their seats would go on the vulnerable list when Tommy Thompson said he was interested in running for the Senate last week," said PPP's Dean Debnam. "But Thompson's popularity is not what it used to be, and it looks like Russ Feingold is in a solid position."