Sen. Bill Nelson is “ripe for a strong challenge” in 2012, according to polling released today that gives Florida’s senior Senator mediocre job approval numbers and suggests that a handful of potential GOP challengers are within striking distance.

Still, the Democratic firm Public Policy Polling found that Nelson, a Democrat, leads every head-to-head matchup save one. Only former Gov. Jeb Bush, who has indicated he will not run for the Senate in 2012, leads the two-term incumbent. None of the others surveyed come within 8 points, based on a sample of 1,034 Florida voters polled Dec. 17 to 20.

Bush leads a hypothetical matchup, 49 percent to Nelson’s 44 percent. Among the more likely candidates, Nelson leads Rep. Connie Mack IV 44 percent to 36 percent, outgoing appointed Sen. George LeMieux 47 to 36 percent, state Senate President Mike Haridopolos 44 percent to 32 percent, and state Rep. Adam Hasner 46 percent to 30 percent.

“Right now, it appears unlikely candidate Jeb Bush is the only Republican who could defeat Bill Nelson,” Dean Debnam, president of Public Policy Polling, said in a statement. “But Nelson’s job performance reviews are not stellar, and Marco Rubio showed this year that a candidate can come out of nowhere to beat the odds.”

PPP says that Nelson is actually a bit better off than his numbers suggest, primarily because of relatively strong support from Republicans and independents. While just 36 percent of all voters approve of his job performance compared with 33 percent who disapprove (a whopping 31 percent have no opinion), an unusually large bloc of Republicans, 23 percent, and 42 percent of independents approve of his performance.

“23 percent might not sound like a lot of crossover support but many Democrats this year are finding themselves with single digit approval with Republicans,” pollster Tom Jensen notes in a blog post about the numbers. 

Jensen also finds that an unusual number of Republicans and independents are willing to vote for Nelson in matchups against established Republicans. For example, Nelson leads Haridopolos and Mack by 27 points with independents and has a 28-point advantage over Hasner and LeMieux with that group. He also gets between 15 percent to 22 percent of the GOP vote against each of them.

Of course, the second-tier Republican challengers hardly enjoy widespread name recognition at this stage of the game. Their numbers across the board could significantly improve following an aggressive and well-funded campaign. Republicans say they will contest this seat heavily given Rubio’s successful bid for the open Senate seat this year.

So far, none of the rumored Republican challengers has formally announced a challenge, but Haridopolos visited Washington late last month to help raise his profile. 

“I’d bet that we’d be in the race,” Haridopolos told the Orlando Sentinel of the 2012 Senate contest at the time. “We’re talking to folks not just across Florida but across the country about that prospect.”

Roll Call Politics rates this race a Tossup.

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