Rasmussen on the Wisconsin primary: Romney, 46%, Santorum 33%, Paul 8%, Gingrich 7%.

Ah! I hadn't seen a poll more recent than last February for Wisconsin, and Santorum had been in the lead. This surveys "Likely Republican Primary Voters in Wisconsin favor Romney" (but in Wisconsin you don't have to be a Republican to vote in the Republican primary):

Romney leads Santorum 57% to 24% among voters who emphasize a candidate who can beat Obama. Santorum leads Romney by a much narrower 43% to 35% among those who prefer a candidate who best represents GOP values....

Among Wisconsin Republicans who describe themselves as Very Conservative, Santorum edges Romney 43% to 39%. Romney leads his chief rival by roughly 20 points among voters who are Somewhat Conservative or not conservative.

Romney leads among both Tea Party voters and non-Tea Party members in the state. Santorum holds a small lead among Evangelical Christian voters, but Romney is well ahead among other Protestants, Catholics and voters of other faiths in Wisconsin.