Rick Santorum leads Mitt Romney in Michigan by widely varying margins in recent polls, but a potentially decisive question mark lingers ahead of the Wolverine State’s Feb. 28 presidential primary: How many political independents will show up to the polls?

Overall, Santorum edges Romney by four points in a poll released Thursday by the Glengariff Group for the Detroit News and television stations WDIV and WZZN, an advantage that is within the margin of error. A flurry of automated and live interviewer polls conducted in the past week show Santorum leading by as much as 15 points and as little as three, high variance for polls conducted within the span of a week.

Republican presidential candidate and former Massachusetts governor Mitt Romney laughs with supporters as a child cries after he tried to hold him at a campaign rally in Kentwood, Mich., on Wednesday. (Gerald Herbert/AP)

This year, Romney leads Santorum by 10 points among political independents in the Detroit News poll, but is trailing by the same margin among rank-and-file Republicans, according to pollster Richard Czuba. His overall deficit is attributable to the fact that independents made up only one in four likely voters in the sample, which is similar to their share of 2008 primary voters.

The Republican vs. independent split is even larger in a poll by American Research Group (ARG) released Monday. Notably, several automated polls show Romney doing no better among independents.

Michigan’s independents were a weak spot for Romney four years ago, when he won 29 percent compared with 41 percent of self-identified Republicans.

Who can participate, and will they?

Michigan is effectively an open primary and is open to Republicans as well as independents and Democrats who choose a Republican ballot at their polling place. (Fun fact: Michigan labels its primary a “closed” event, but this just refers to the fact that voters can’t participate in both Republican and Democratic events).

Independent turnout could be boosted or depressed by changes since 2008. The absence of a Democratic contest could lead to higher independent participation in the GOP primary. But new voting disclosure rules could make voters leery of casting a ballot. In compliance with a 2008 lawsuit, the secretary of state will make voters’ party preference in the primary public after the election (see page 15 here).

Romney could also benefit from higher turnout overall. Romney topped Santorum by 11 points in the ARG poll among voters who ranked their likelihood of voting between “7-9” out of 10. But among voters who ranked themselves as “10,” Santorum led Romney by 11 points.

This trend appears the opposite of what occurred in the Florida and Colorado contests, where Romney’s challengers performed better in counties where turnout was up from 2008. (Scatter plot hat tip to Michael McDonald and the Enik Rising blog)

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