Though Michigan and Arizona will award roughly the same number of delegates tonight (with both losing half their original allocations for non-compliance with RNC calendar rules), and though Arizona will provide the larger “purse” by virtue of a statewide winner-take-all system, Michigan will get the lion’s share of attention simply because it is by far the closer race, and is Mitt Romney’s native state. The major drama will be all about Rick Santorum’s bid for an upset there.

All the late polls in MI show a very close race, with Santorum’s original lead yielding to a TV-fed Romney surge, and then a Santorum comeback, apparently on the strength of Democratic and independent voters (with the former including at least some “tactical” or mischief-making ballots cast by people who wouldn’t normally support Rick for dogcatcher).

It’s Mitt Romney’s apparently large lead among early voters that gives him some confidence in MI despite doing poorly down the stretch, as Tom Jenson of PPP explains:

Even though things seem to be moving back in Santorum’s direction, there’s one big reason to think that Romney will still come out as the winner tomorrow night. 18% of the electorate has already cast its ballots and with those voters in the bank Romney has a 56-29 advantage. Santorum’s likely to win election day voters, but he”s going to have to do it by a wide margin to erase the lead Romney has stored. We see Santorum with a 40-33 advantage among those who have yet to vote.

Romney hasn’t made a good last impression on Michigan voters. His favorability in Sunday interviews was 57/36, but in Monday interviews it was only 47/48. Santorum saw little difference in his reviews between the two days: 54/39 on Sunday and 56/36 on Monday. If Romney does indeed end up losing tomorrow there’s not much doubt he will have blown it in the final 48 hours.

Nate Silver, whose forcasting model gives Romney a 55% probabiity of victory in MI, says it’s really “too close to call.”

We’ll have more on these primaries later.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.