If polling is accurate, former Massachusetts governor Mitt Romney is headed for a three-state sweep tonight.

(Screenshot from the PostPolitics iPadd app.)

Forty-two delegates are at stake. Fifteen of Wisconsin’s delegates are allocated proportionally, and 24 are doled out by congressional district.

(That Romney will primaries in Maryland and D.C. is not really in doubt. Santorum isn’t even on the ballot in the District.)

This chart shows that while Santorum is trending upwards slightly, he has a big gap to close:

But low enthusiasm in Wisconsin could hurt Romney. The looming gubernatorial recall election is getting far more attention than the presidential race, and Santorum has a history of performing better on primary day than he does in polls.

So while Romney is still on track to win Wisconsin, his margin of victory will likely be slimmer than polls would suggest. There’s even a chance Santorum could pull off an upset.