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Rick Santorum has emerged as a much more popular figure with the party’s conservative base in Wisconsin than his GOP rivals, according to a new statewide poll.

But all four Republican presidential candidates trail President Obama by double-digits in a Marquette Law School survey of 716 registered voters taken Sunday through Thursday.

Obama, who last week made his first 2012 trip to Wisconsin, leads Santorum here 51% to 40% in a general election matchup. He leads Mitt Romney 53% to 38%. He leads Ron Paul 52% to 36%. And he leads Newt Gingrich 56% to 33%.

Those numbers coincide with a slight uptick in Obama’s popularity since last month and an erosion of support for Romney and Gingrich, two big-name Republicans who’ve taken on baggage in their party’s bruising, topsy-turvy nominating fight.

Both Romney and Gingrich have seen their numbers worsen among Republicans since Marquette’s previous poll in January. And both trail Santorum, the former Pennsylvania senator, by double-digits among likely GOP primary voters in Wisconsin.

Among registered voters who say they plan to vote in the state’s open presidential primary April 3, Santorum leads with 34%, followed by Romney with 18%, Paul with 17% and Gingrich with 12%.

That mirrors Santorum’s rise to the top of the national polls in a GOP race that has defied a tidy resolution, creating at least the chance that the Wisconsin Primary six weeks from now could play a role in the nominating fight.

If that contest were held today, Santorum would find himself in better shape than his three competitors, who have much higher “negatives” among the GOP base.

The chart below shows how Republican voters in Wisconsin viewed their party’s presidential candidates in the February survey:

Santorum’s standing with Wisconsin Republicans has changed little since January. But perceptions of both Romney and Gingrich have grown more negative. Romney’s favorability rating among Republican voters here has dropped from 54% to 46% since last month, and Gingrich’s has dropped from 56% to 40%. Gingrich now has a net negative rating among voters in his own party in Wisconsin, a singular liability in a partisan primary.

Santorum also has a clear advantage among conservative voters. The numbers below are based on the combined results of Marquette’s January and February polls (the polls were combined to get a larger sample size, in this case about 550 “conservative” and “very conservative” voters):

Santorum’s negatives among conservatives are by far the lowest in the field. Romney and Paul are viewed unfavorably by almost as many conservatives as view them favorably. And Gingrich has a net negative rating with Wisconsin conservatives, just as he does with Wisconsin Republicans.

“I think it represents the dilemma of the Republican Primary field that in this extended and very competitive primary process, the candidates are doing damage to each other,” says political scientist Charles Franklin, who did the poll.

Franklin says that doesn’t mean the party won’t rally around the eventual nominee. But for now it has left Santorum in a relatively unscathed category of his own with GOP voters in Marquette’s polling. The question going forward is whether he follows the pattern of other “frontrunners” in the race who’ve been damaged by their opponents’ attacks and the increased scrutiny that comes with political success. The GOP landscape could change again dramatically between now and early April.

As for Obama, his popularity increased slightly from January to February, though the difference was within the poll’s margin of error of plus or minus 3.7 percentage points. He was viewed favorably by 52% of registered voters and unfavorably by 43%.

One footnote: There was no clear evidence in the poll that the controversy over whether Catholic institutions should have to cover contraception services for employees has hurt Obama among Catholic voters in Wisconsin. The percentage of Catholic voters who viewed Obama favorably was slightly higher in February (48%) than it was in Marquette’s January poll (46%). Catholics made up 33% of the voters polled in the survey.

Here are some other highlights from the poll (full results and methodology are here):

Governor. The fundamentals when it comes to Gov. Scott Walker’s public standing don’t seem to change very much. People are very closely divided and highly polarized along party lines. Walker’s popularity was lower in Marquette’s February poll than it was in January, though the difference was close to the poll’s margin of error. In the new poll, Walker was viewed favorably by 46% and unfavorably by 48% of registered voters. A month ago, he was viewed favorably by 50% and unfavorably by 45%. Voters were evenly split on his job performance, with 47% approving and 47% disapproving. Unlike in January, Marquette did not poll this month on head-to-head matchups between Walker and potential Democratic challengers.

US Senate. In general election matchups, Republican Tommy Thompson led Democrat Tammy Baldwin 48% to 42%, while Baldwin led Republican Mark Neumann 44% to 40% and led Republican Jeff Fitzgerald 45% to 37%.

Issues. The poll asked about the ongoing grand jury investigation in Milwaukee that has led to charges against former Walker aides. About three-quarters of those polled said they were aware of the investigation. Of those who were aware, 52% agreed that it was “really something serious,” while 40% agreed with the statement that it was “just more politics.”

Asked about the proposed iron ore mine in northwestern Wisconsin, 52% supported developing the mine and 33% opposed developing it. (In the wording of the question, voters were told that supporters argue the mine will "create 700 jobs and long-term economic benefits," while opponents argue "not enough environmental protections are in place to preserve water and air quality.")

Jobs. Voters were a little more negative about the recent jobs climate in Wisconsin than they were nationally. Asked if they thought there were more jobs in the state than a year ago, 22% said more, 30% said fewer and 49% said about the same. Asked the same question about the nation as a whole, 31% said there were more jobs, 28% said fewer jobs and 39% said the number was about the same.

Wisconsin generated job growth during the first six months of 2011, but has now lost private-sector jobs for six months in a row, according to the Bureau of Labor Statistics.