Voters in Michigan head to the polls today, carrying the fate of former Massachusetts governor Mitt Romney’s presidential bid in their hands. Win Michigan and, as expected, Arizona, and Romney almost certainly reasserts himself as the clear frontrunner in the Republican race. Lose Michigan and the calls for Romney to reconsider his candidacy will begin. It’s that simple.

View Photo Gallery: Primary voters head to the polls in Arizona and Michigan.

Most polls close in Michigan at 8 p.m. eastern time with a few outliers closing at 9 pm. We will — obviously — be bringing you full coverage both here on the Fix and in a live blog on our Election 2012 blog, which is manned (or womaned) by former Fix deputy Felicia Sonmez.

But, in the meantime, we thought it made sense to provide you, loyal reader, a viewer’s guide/CliffsNotes of what to watch for as you watch the results. We chatted with a handful of veteran Michigan Republican operatives to find the five counties that will tell us something important about what’s happening in the Wolverine State. The counties are listed below in alphabetical order. Enjoy!

* Kent County: Western Michigan is the home to social conservatives in the state and, as such, should be strong territory for former Pennsylvania senator Rick Santorum. But, Kent includes the city of Grand Rapids and its affluent suburbs, which are packed with busi­ness­peo­ple jonesing to beat President Obama in the fall. And, as a further complication for Santorum, Rep. Justin Amash, the freshman Republican who represents Kent in Congress, has endorsed Texas Rep. Ron Paul. If there is a county in Western Michigan that Romney can win, it’s Kent.

* Macomb County: The northeastern suburb of Detroit is the home to the Reagan Democrat — blue-collar voters who went for Republicans in the 1980s but have moved toward Democrats in subsequent elections. Romney won Macomb by almost 16,000 votes over Arizona Sen. John McCain in the 2008 Michigan primary but Santorum is running hard as an economic populist while the former Massachusetts governor has made a number of impolitic comments about his own wealth. Romney’s difficulties in connecting with working-class voters could come home to roost in Macomb.

* Marquette County: The U.P. — Upper Peninsula — of Michigan is not a place that sees all that many presidential candidates. It’s hard to get to and there just aren’t that many votes in the far northern counties of the state. (Marquette is the largest city in the U.P.- based 1st district and its population is just 21,000 or so.) But, Santorum made a series of stops in the U.P. on Sunday — he appeared on NBC’s “Meet the Press” between those stops — and touted the similarities between the U.P. and where he grew up in Western Pennsylvania. This was also one of the counties where McCain beat Romney in 2008, suggesting Yoopers don’t have any special affection for their native son.

* Oakland County: When asked to name five counties worth paying attention to tonight, one Republican operative said: “Oakland, Oakland, Oakland, Oakland and Oakland”. It’s that important — particularly to Romney’s chances of winning the state. Oakland is the wealthy Detroit suburb full of the sort of fiscally conservative, socially moderate Republicans that must be Romney voters — and in big numbers. There’s no debate about the fact that Romney will win Oakland but the size of that victory and what percentage of the statewide vote Oakland accounts for is critically important to his chances. In 2008, 28 percent of the overall vote came from Oakland County; one in every five people who voted for Romney came from Oakland.

* Wayne County: The sexiest story going into today’s vote is whether — and in what numbers — Democrats will turn out to participate in the Republican primary. (The state holds an open primary meaning that Democrats are free to cast votes on the GOP side without penalty.) If there is a county where that crossover vote will be seen (or not) it’s Wayne in the southern suburbs of Detroit. Wayne is giant population- wise — one in every three votes in the 2008 Republican presidential primary came from the county — and Romney won it by more than 18,000 votes in 2008. The county, which inclues the city of Detroit, is still strongly Democratic in general elections, however, as President Obama won it with 74 percent against McCain in 2008. If Santorum is running close to Romney in Wayne, it’s almost certainly the result of Democrats crossing over to disrupt the former Massachusetts governor’s path to the Republican nomination.

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