Mitt Romney leads Rick Santorum in Wisconsin poll

Mitt Romney has opened a solid lead over Rick Santorum in Wisconsin's Republican presidential primary, a big swing from a month ago.

A Marquette Law School poll released Tuesday found Romney leading Santorum among likely GOP primary voters by 8 points—39 percent to 31 percent. Ron Paul trailed with 11 percent support, followed by Newt Gingrich who registered at just 5 percent.

That's a 21-point gain for Romney in just a month. In February, the same poll found Santorum leading Romney 34 percent to 18 percent.

The poll comes just days before Tuesday's GOP primary, which is considered a must-win for Santorum, who is trying to keep pace with Romney in the race for convention delegates. But it could be an uphill battle.

Unlike recent contests, Wisconsin is a "winner take all" primary, which means all of its 42 delegates will go to the candidate who wins the popular vote. Romney is already heavily favored in the two other contests scheduled for next Tuesday: Maryland and the District of Columbia, where Santorum failed to qualify for the ballot.

But there are hints of a potential upset if Santorum can gain momentum. While Romney has been pushing the electability argument on the trail—insisting he's in the best position to beat President Barack Obama—just 19 percent of those polled said the ability to defeat Obama is driving their vote. Rather, 32 percent said "strong moral character" was the most important issue to them, followed by 30 percent who cited "experience."

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