Wisconsin, like my home state of Michigan, doesn’t have party registration. Any registered voter can walk into the polls and vote in either the Republican or Democratic primary. In 2008 there were serious contests in both parties in Wisconsin. In 2012 there’s a serious contest only in the Republican primary. According to the exit poll, 30% of Republican primary voters identified themselves as Independents and 11% as Democrats. Among self-identified Democrats, Rick Santorum beat Mitt Romney 37%-19%. That amounted to a Santorum popular vote majority of 2% of the total vote. You might want to keep that in mind in interpreting the statewide percentage. Among self-identifed Republicans, Romney won 51%-37%. That's pretty conclusive about what Republicans want.
Look at a couple of other results. Rick Santorum, the champion of opposite-sex marriage, carried the 30% of primary voters who are unmarried by 37%-32%. I’m guessing (and am pretty sure about my guess) that the great bulk of crossover Democrats are unmarried. Santorum also carried (and Ron Paul ran second among) the small number who said Romney was too conservative; he carried, narrowly, those who say the economy is getting better (which includes all those Democratic crossovers); he carried those who disapprove of Governor Scott Walker. In other words, Romney consolidated the Republican vote in Wisconsin, and Santorum’s total was boosted by crossover Democrats who want the Republicans to nominate what they believe to be a candidate who would be easy to defeat.
We saw some crossover voting in Michigan, which doesn’t have party registration, and in Ohio Santorum’s strongest single county was Athens County, home of Ohio University, which is situated in the midst of Appalachian-type culturally conservative counties. Why did Athens County vote so heavily for Santorum? A probably correct guess: a town-gown alliance, a combination of Appalachian-type cultural conservatives plus college town, mischief minded lefties crossing over into the Republican primary.
In contrast, Romney carried very conservative voters and strong tea party supporters and lost white evangelical Protestants to Santorum by only 41%-39%. In Wisconsin Santorum’s vote was boosted by liberals who hate Republicans. Romney’s margin understates the extent to which he is the candidate of Wisconsin Republicans.