Gerald Herbert - AP Photo / Eric Gay - AP Photo

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.

That's off their peak - 17 percent in 2000, when Democrats tipped the open primary to John McCain - but potentially enough to influence the outcome.

Ideologically, six in 10 voters in Michigan today describe themselves as conservative, evenly split between "very" and "somewhat" conservative. Preliminary exit poll results indicate that very conservatives, a group on which Santorum has focused his efforts, may account for a slightly larger share of the electorate than in 2008, when they made up one in four Michigan GOP primary voters.

If that helps Santorum, he's challenged by the share of evangelicals in the Michigan electorate - about four in 10, similar to their 2008 levels and lower than their share in two states in which Romney's opponents have run especially well this year: Iowa (very narrowly won by Santorum) and South Carolina (easily won by Newt Gingrich).

Also worth watching are the choices of voters looking for a candidate who shares their religious beliefs; more than half say this matters to them, although many fewer, about a quarter, say it matters "a great deal"; and the six in 10 who oppose legal abortion, with differences between those who favor making it illegal in all cases, as opposed to most cases.

Seniors are another group to watch. Preliminary exit poll results find them turning out in far larger than customary numbers for the Michigan primary, albeit about usual for other states this year. Elsewhere it's been a good group for Romney.

One other result suggests the GOP electorate in Michigan is not exactly fired up: Just under half of voters say they strongly favor their candidate. Four in 10 like their guy but with reservations, while the rest mainly dislike the other candidates.

Among other groups:

PARTY: 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.

QUALITIES: A third of voters say they're most interested in electability - the candidate with the best chance to defeat Obama. That has been a strong Romney group in previous contests. But two other attributes are close behind: experience and strong moral character, each picked by about a quarter of voters. In a result potentially unhelpful to Santorum, fewer - about one in seven - say they're looking mainly for the true conservative in the race. And in a separate question, slightly more than half see Romney as most likely to defeat President Obama in November, double the number who say so about Santorum.

UNION and BAILOUT: Just under a quarter of voters are from union households, continuing a downward path from 2004 and 2008 in their share of the electorate. Romney did better among non-union than among union voters in 2008, and tried to tie Santorum to "big labor" this year.

More GOP voters in the state disapprove than approve of the federal government's automaker bailout, but not by a wide margin - 51-43 percent, perhaps a surprising level of support in this particular population, given the party's customary laissez-faire approach. Romney and Santorum both opposed the bailout.

ISSUES: More than half of Michigan voters say the economy's the most important issue in their vote, by far the top rated issue; just under a quarter named the federal budget deficit; one in seven pick abortion, with illegal immigration in the low single digits. Also on the economy, three in 10 had a job loss or layoff in their household in the past three years.

At the better-off end of the spectrum, three in 10 voters report household incomes over $100,000 a year, another group that's been good for Romney in previous contests this year.

DEBATE/DECISION: About a third of voters call the last pre-primary debate important in their vote today. And about a quarter decided in the last few days, down from a third who said so in 2008. That means three-quarters decided earlier, a group that in most previous contests has tended to be a better one for Romney.