The polls close in most of the state at 8 p.m. ET but a few precincts in the upper peninsula will keep going until 9 p.m. The good news: For the first time since Iowa, we’ve got a major election whose conclusion isn’t foregone. The bad news: The fallout tomorrow is wholly predictable no matter what happens, barring an exceedingly unlikely Romney landslide victory. As long as it’s close, the spin will be that Mitt struggled in his “home state,” that he’s a paper tiger, and that the GOP establishment needs to rethink its support for him as Super Tuesday looms. (For a sneak preview, try this.) Obviously the din will be louder if Santorum wins outright, but even a narrow Romney victory — and it looks to be very narrow if it happens — will be grounds for seven more days of “weak frontrunner” Romney-bashing. Never mind that he’s poised to utterly crush Santorum in the other big primary tonight.

The counterspin tomorrow from Team Mitt will take two tacks. One: Don’t worry about the popular vote, worry about the number of delegates. Mitt doesn’t care if 60-65 percent of primary voters are consistently divvied up among the Not Romneys. You don’t have to like him; you don’t even have to respect him. All you have to do is nominate him. 1,144 delegates or bust. Two: The damned dirty Democrats rigged the vote for Santorum. And you know what? Maybe they did:

A divided Republican base is defining the Michigan GOP primary today, with Mitt Romney and Rick Santorum again playing to competing wings of the electorate. Santorum, in particular, is seeking advantage among strongly conservative voters. But perhaps controversially, one in 10 voters in the open primary are another stripe entirely – Democrats…

Six in 10 voters in today’s Michigan Republican primary describe themselves as Republicans, about 10 points fewer than in 2008. (But it’s been lower, just 48 percent in 2000, and also was lower this year in New Hampshire.) Independents account for three in 10 voters, Democrats, as noted, one in 10. In past contests Romney generally has done better with mainline Republicans than with non-Republicans.

Democratic shenanigans at work or Team Santorum outreach? The votes count either way, and if RS does end up winning big among Dems — by 35 points!, according to CNN’s exit poll — he’s got cover from Rush Limbaugh on robocalling them. (Fun fact: Santorum complained about Democrats voting in Republican primaries as recently as three weeks ago.) Via the Examiner, watch the clip below of Michael Moore from last night’s Maddow show. Can you feel the Santorumania?

Mitt’s talking tough today, predicting victory and insisting that it won’t come to a brokered convention, but one exit poll has him leading by one thin point and another poll taken last night put Santorum up by five. Put on some coffee because we’ll be up late. Here’s your Google/AP results page for following along; lots of updates coming, needless to say. While we wait, two questions to chew on. First, why on earth did the RNC decide to roll out a proportional system of awarding delegates in a cycle when they’re pitted against a Democratic incumbent? The time to do that was in 2008, when both parties were consumed with primaries. As it is, we’re looking at a long internecine battle while Obama sits back and piles up money. And second, what, if anything, could convince Romney to drop out? If he underperforms on Super Tuesday, would that do it? What about the primaries after that? I find myself wondering more and more why he’s so determined to win when he receives so much negative feedback at every turn. He has few passionate supporters and many passionate detractors; he has no big cause or grand issue that animates him; his victories are owed chiefly to carpet-bombing his rivals with negative ads rather than stirring up enthusiasm for his candidacy. It’s almost a test of wills with the base, or some sort of exceptionally complex organizational problem he’s determined to solve. Is Mitt so skillful a manager that he can propel a candidacy built on virtually nothing to the Republican nomination despite resolute opposition from activists? Stay tuned!

Update: Speaking of tomorrow’s spin, read Mark Halperin’s amusing and accurate 12-point list.

Update: Am I mistaken or has the nastiness between Romney and Santorum spiked in the past 24 hours? Mitt was unusually harsh in his grumbling about RS’s robocalls to Democrats and now here’s Santorum calling him a “lightweight” and a bully. Quote: “That’s what bullies do. When you hit them back, they whine.”

Update: The first good sign for Santorum tonight comes from Nate Silver, who notes that if the exit polls are right, the percentage of indies and Democrats voting today is higher than pollsters have estimated. If that’s true, and if it’s also true that RS is winning big among Dems, then the polls may be lowballing him.

Update: Philip Klein does the back-of-the-envelope math and says Democratic support could be worth as much as 3.5 points to Santorum tonight.

Upate: Some rocket fuel for tomorrow’s “GOP chaos!” post-Michigan spin from RCP elections guru Sean Trende. If current trends in Super Tuesday states hold, the odds of a brokered convention will increase considerably by next Wednesday:

What’s interesting is that from Super Tuesday forward, only 1,580 delegates remain. This means that Romney would have to win 50 percent of the remaining delegates, Santorum would have to win 58 percent, and Gingrich and Paul need around two-thirds of them to reach a majority.

Now, in theory, this should be easier for Romney to do: 434 delegates would be awarded in the South, 389 in the Midwest, 89 in the Mountain West, 194 on the Pacific Coast (including 169 in California), 244 in New England, and 230 in other places (RNC delegates and territorial delegates).

When you consider that a lot of the New England and Pacific states are winner-take-all (or some variant of that), while the Southern and Midwestern states are proportional, Romney’s path becomes clearer.

But he will have just taken a drubbing on Super Tuesday. The headlines will be terrible, which may put downward pressure on his polling numbers in New England or in the Mountain West. That would help Santorum, but winning nearly 60 percent of the remaining delegates is a tall order for him, especially with Gingrich and Paul gobbling up delegates here and there.

Trende now gives a brokered convention a 20 percent chance of happening.

Update: With nearly 10 percent of precincts reporting, Santorum leads by one point. Just as I’m writing that, meanwhile, news is breaking that a new debate has been added to the schedule. It’s this Saturday — on Mike Huckabee’s Fox News show. Romney, Santorum, and Gingrich have all confirmed; Paul is still undecided.

Update: 9 p.m. ET is here bearing no surprises. Michigan is too close to call but Romney romps in Arizona. 29 more delegates for Mitt. Jim Geraghty estimates that even in a worst-case scenario for Romney, where Santorum narrowly wins Michigan, he’ll end the night with roughly 43 delegates compares to 16 or so for RS. There’s Team Mitt’s lead talking point tomorrow morning.

Update: Suddenly, with nearly 20 percent reporting in Michigan, Romney’s up by more than three points and 8,000 votes. He’s way, way, way up on InTrade too, above 90 percent odds to win after being stuck around 50 percent earlier. Romentum?

Update: Here’s the Michigan exit poll via CNN. If it holds up, Romney wins tonight by a few percentage points.

Update: The most interesting data point from the exit poll:

Note that nearly half said they made up their minds before New Year’s. Despite Santorum’s huge surge among more recent deciders, Mitt crushed him enough among early birds that he should be able to hold on and win the state. (Intriguing footnote, though: Among those who decided today, Romney won 38/31.) Another odd data set:

Santorum crushed Romney among Democrats and fought him to a draw among independents — yet somehow, the “conservative” candidate lost Republicans by 10 full points. If RS ends up losing tonight, that’ll be the headscratcher tomorrow. How did he fail so badly against Romney within the party itself that he let the state slip away?

A few other random points. Santorum narrowly won every age demographic except seniors (25 percent of the electorate), among whom Romney won by 17 points. Santorum beat Romney by nearly 20 points among union members but lost by five points among the heavy majority that don’t belong to a union. Romney won by four among self-described moderates and liberals — and by three among self-described conservatives. A plurality of 32 percent said electability is the most important quality in a candidate, and among that group Romney wins 61/24.

Update: I’m hearing on Twitter that with 30 percent of precincts reporting, Karl Rove said on Fox that he thinks Romney will hold on to win. As I write this, Romney leads by three and a half points and more than 12,000 votes. That’s a decent margin for this late in the tabulations. I wonder how long it’ll be before we get a call.

Update: Fox News has a more complete exit poll. Devastating:

I’m dying to see the crosstabs on when Catholic voters decided. Did the attention given to Santorum’s criticism of JFK’s church-and-state speech this weekend possibly alienate some of them? (Note again that Romney won among voters who decided today even though Santorum crushed him among voters who decided over the past two months.) Or is something else going on? Liberal Catholics maybe reacting to Santorum’s contraception rhetoric? Theories?

Update: Dave Weigel finds a silver lining for Santorum in tonight’s numbers. If you toss out Romney’s big advantage in the Detroit area, the demographics bode well for Ohio.

Update: Nearly 45 percent reporting now and Romney’s lead is getting bigger. He’s up by four percentage points and more than 20,000 votes. Tick tock.

Update: With 62 percent in, Romney’s now leads by more than 30,000 votes and is still piling ’em up in Detroit’s county. Lights out.

Update: No sooner did I publish that than, at 10:14 ET, NBC calls Michigan for Romney. Santorum is quasi-conceding right now. Looks like Mitt will win by roughly five points. Realistically, given the recent polling, that’s the best possible result he could have hoped for tonight, good enough to blunt the media’s attempts to spin this as a moral victory for RS even if Mitt did need to outspend Santorum by two to one or so just to hold his home state.

Update: Romney’s doing so well tonight, in fact, that he might end up with a bonus talking point tomorrow. In 2008, he won the state with 338,000 votes. Tonight, with a little more than 70 percent reporting, he’s north of 306,000. If he outdoes his total four years ago amid lower turnout, he’ll have some dynamite spin for the naysayers.

Update: One of Romney’s advisors floats another bit of effective spin on Twitter by noting that he’s the only candidate to win primaries in the northeast (New Hampshire), southeast (Florida), southwest (Arizona), and now the midwest.

Update: With just 82 percent reporting, he’s already at 345,000 votes, several thousand greater than his take in 2008. Expect to hear a lot about that from his team tomorrow.