Michigan's primary drew the Republican Party's ideological, religious and socioeconomic divisions in sharp relief, raising questions both for the primaries ahead and for the party's ability to coalesce behind its eventual candidate.

Mitt Romney won the event, relying especially on older and wealthier voters to turn back a stiff challenge from Rick Santorum. But it was voters of an unusual stripe for a Republican primary - Democrats - who made things especially interesting.

In the closest primary to date this year (excluding more lightly attended caucuses), Santorum easily won the groups he's targeted, including strong conservatives, evangelicals, strong Tea Party supporters and ardent abortion opponents. Union voters, fleeing Mitt Romney, helped Santorum. And so did Democrats.

Exit poll results found that nearly one in 10 voters in Michigan's open primary were Democrats. That was off their peak - 17 percent in 2000, when they tipped the contest to John McCain. But they influenced this year's outcome nonetheless: Santorum won 53 percent of Democrats, versus just 17 percent for Romney. Without them Romney would have had a fairly comfortable win. With them it was closer.

Romney relied on some of his customary support groups, notably well-off voters and senior citizens - effective elements, if not necessarily the most compelling ones for a national campaign. He won senior citizens, but no other age group. He won $100,000-plus voters, but no other income group. He especially won $200,000-plus voters, a group around which it's hard to build a slogan for the masses.

Indeed, in a result suggesting that Romney won more heads than hearts, he beat Santorum by a 2-1 margin, 52-26 percent, as the candidate seen as most likely to defeat Barack Obama in November. Yet the contest between the two in this primary was a far closer one than that.

Another finding revealed a less-than-fired-up electorate. Fewer than half of voters, 45 percent, said they were strongly behind their candidate; more instead said they either liked their man with reservations (38 percent) or chiefly disliked the other guy (15 percent). Just half of Romney's supporters, and fewer of Santorum's - 38 percent - strongly favored the candidate they voted for.

In the night's other contest, Romney had a far easier ride in Arizona, where his campaign was largely unanswered by Santorum. Romney ran competitively in the groups in which he's shown vulnerability this cycle - and that he lost in Michigan - and dominated in other, less ideological groups. He also benefited from the largest share of early-deciders this cycle.

In Michigan, after painting Santorum as a tool of "big labor," Romney lost voters from union households to him by 27-45 percent. That may have been intentional; they've made up a slipping share of Michigan GOP primary voters (down from 34 percent in 2000 to 23 percent this year), and Romney appeared to reach instead for the larger non-union vote. He won it - but by just an 8-point margin.

Ideologically, about six in 10 Michigan voters described themselves as conservative, evenly split between "very" and "somewhat" conservative. Very conservatives went to Santorum by 50-35 percent; their less ideological brethren, to Romney by an almost identical margin, 50-31 percent. Moderates made the difference, presumably not the group most Republicans prefer to have pick their nominee.

Santorum won evangelicals, a group among which Romney, a Mormon, has struggled, by a wide 50-32 percent; along with very conservatives, it's another potentially ominous result for Romney in the Southern primaries ahead: In 2008, evangelicals accounted for 73 percent of GOP primary voters in Tennessee, 72 percent in Oklahoma and 62 percent in Georgia, all among next week's Super Tuesday states.

Santorum, similarly, won voters who care a "great deal" that a candidate shares their religious beliefs, by a vast 42 points, and those who want abortion illegal in all cases, by a nearly as vast 33 points. But he has his own challenges in these results - voters with less emphatic views on those two questions went for Romney.

While Romney may have pushed away voters from union households, Santorum, in positioning himself as the working-class candidate, may not have done himself favors with college-educated voters. They favored Romney by 44-36 percent, and accounted for 51 percent of primary voters, 8 points more than in 2008. And while Santorum won voters with less-than $50,000 incomes, it was by just 5 percentage points. Those with incomes of $100,000 to $200,000 went for Romney by 7 points; with incomes of $200,000 or more, by a wide 26 points, 55-29 percent.

The divisions among candidate attribute groups were among the sharpest of all. About a third of Michigan voters said they were most interested in electability - the candidate with the best chance to defeat Obama; again a strong Romney group, he won them by a resounding 61-24 percent.

But another attribute was not far behind; having "strong moral character" was selected by 25 percent, and they went for Santorum by an equally wide 57-16 percent. Santorum also won the one in six mainly seeking the "true conservative" in the race, by an almost identical margin, 57-17 percent. And the one in five most looking for the candidate with the best experience turned the other way, 56-14 percent for Romney. These gaps - 37, 41, 40 and 42 points - starkly draw the Romney/Santorum battle lines ahead.

One final result may provide ammunition for yet another combatant, Barack Obama. More GOP voters in the state disapproved than approved of the federal government's bailout of the auto industry - but the margin, 51-44 percent, was not a wide one. Romney and Santorum both opposed the bailout. If it's popular with 44 percent of Republican primary voters in Michigan, Obama - who highlighted the rescue in a speech to the United Auto Workers on Tuesday - is likely to bring it up early and often.