Michigan's primary today is the first to be held in the U.S. Rust Belt, a contest that presents fresh opportunities for candidates in both parties to make a noise in what's often considered a bellwether state. 

For Democrats Hillary Clinton and Bernie Sanders, it's a day that will test how their messages on trade and manufacturing resonate in the industrial Midwest, and a chance to gauge whether Sanders has enough gas in the tank to proceed. 

For Republicans, it's a chance to show whether a moderate conservative candidate like Ohio Governor John Kasich can challenge front-runner Donald Trump.

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Ronna Romney McDaniel, the current chair of the Michigan Republican Party, says Michigan is a state with a political makeup that 'encapsulates the whole country in one location.' (Jae C. Hong/Associated Press)

Michigan polls show the Ohio governor continuing to gain ground on Trump. This week, an ARG poll had Kasich ahead of Trump by two points, 33 per cent to 31 per cent. The latest Monmouth University poll had Trump with a 36 per cent lead, followed by Ted Cruz at 23 per cent, then Kasich in third place with 21 per cent.

"This is a new phase," says Denise DeCook, a Republican strategist with the consulting firm Sterling Corp. in Lansing, Mich. 

"Kasich is kind of like a favourite son here. He speaks our language. And if Kasich comes out first or second in Michigan, just a week out of Ohio, that continues to deny Donald Trump from getting the nomination."

If Trump wins, on the other hand, it would show his appeal is broader than many have given him credit for.

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Bernie Sanders speaks to the crowd during a rally at Wings Event Center Monday, March 7, 2016 in Kalamazoo, Mich. Sanders's message condemning trade agreements and blaming them for costing American jobs could resonate with Michigan voters who have suffered through a decimated manufacturing economy. (Bryan Bennett/Associated Press)

In either scenario, much depends, of course, on Michigan's "purple state" politics, meaning an electorate that neither veers hard right, nor hard left.

"We are a state that votes primarily Democratic in presidential years, and primarily Republican in gubernatorial years," says Ronna Romney McDaniel, the niece of former GOP nominee Mitt Romney and the current chair of the Michigan Republican Party in Lansing. 

"We have agricultural, we have manufacturing. We really are a state that encapsulates the whole country in one location," McDaniel says.

More blue-collar voters

Today's election comes a week before March 15 — the so-called mini-Super Tuesday — when states voting in Republican primaries switch to winner-take-all contests, rather than proportionally allocating delegates.

 If you can win in Michigan, you're talking about the issues in the right way, and you're appealing to voters in the right way. - - Michael Traugott, politics professor at the University of Michigan

Next week, Kasich is favoured to win all 66 of Ohio's delegates. A strong Michigan showing tonight could give him enough thrust to reposition himself as the more staid alternative to Trump.

"Trump has been [polling] at around 40 per cent pretty much from the beginning, but it's the next 10 to 11 per cent of delegates that will be the hardest he's ever had to fight for," says DeCook, who doesn't work for a particular campaign.  

Texas Senator Ted Cruz, a Tea Party favourite, would be able to boost his viability as a contender should he do better than expected in Michigan. 

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Some political observers say the Democrats are taking a closer look at everything to do with Donald Trump because of his success in the primaries. (Jim Young/Reuters)

But for a candidate with a more moderate brand of conservatism, the road to securing the nomination will have to run through the Wolverine State. 

Beyond Michigan are more blue-collar voters in Ohio, Missouri, Illinois and Wisconsin. 

The primary calendar, DeCook says, is now shifting towards voters who have "a much more discerning" view of how a president should behave. 

Michiganders don't only want someone who can govern, she believes, but "someone who can govern with respect."

In other words, they may not be so easily sold on the Trump persona.

"I believe that's something that's true for Ohio, Illinois, Indiana, Pennsylvania — all states that haven't been tested yet," she said.

Testing ground for the fall

Jobs will be high on the minds of Michigan voters, as the state crawls back from economic downturn and a devastated manufacturing industry.

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Democratic presidential candidate Hillary Clinton is expected to win in Michigan, though her rival Bernie Sanders could blunt her lead with his repeated attacks on her support for 'disastrous' trade policies, a theme that may resonate with Michigan voters. (Rhona Wise/AFP/Getty Images)

"The state is a testing ground for issues that will be important in the fall," says Michael Traugott, a politics professor at that University of Michigan's Institute for Social Research.

"That includes manufacturing and trade, but also the pace and strength of the economic recovery, so a concern about the middle class plays very well with the Michigan electorate."

On the Democratic side, Sanders's campaign manager Jeff Weaver has called Michigan "a critical showdown," in part because of the Vermont senator's economic message condemning "unfair" trade pacts, specifically the North American Free Trade Agreement signed into law by Clinton's husband.

It's the kind of criticism that should appeal directly to Michigan voters. 

A good audience for Sanders

Sanders has argued that "bad" trade pacts have cost "millions of decent paying jobs" for Americans, a theme that is deeply felt in the Rust Belt.  

But Clinton is expected to win in Michigan, and was holding a 13-point lead on Sanders, according to a Monmouth University poll on Monday. 

Clinton has maintained a strong presence in Michigan, and reached out to the city of Flint amid its lead-poisoned water crisis. 

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Republican U.S. presidential candidate Donald Trump displays his hands as rivals Marco Rubio, Ted Cruz, and John Kasich look on during the U.S. Republican presidential candidates debate in Detroit, Michigan. (Jim Young/Reuters)

In January, Flint mayor Karen Weaver endorsed Clinton's candidacy, saying the former secretary of state was the only candidate from either party who offered to assist during the crisis.

"The issues that are brought up here — water, trade, manufacturing — those will resonate in all those [Rust Belt] states," says Mark Brewer, the former chair of the Michigan Democratic Party.

"Michigan is a bellwether state. If you can win in Michigan, you're talking about the issues in the right way, and you're appealing to voters in the right way."

Sanders laid into Clinton during the Flint debate over her support of "disastrous" trade policies, which he blamed for the loss of jobs in the manufacturing sector.

"The kind of message that Sanders is carrying about Clinton supporting those trade agreements can play very well here," Brewer said, suggesting it might blunt her lead.

If that message fails to connect, a significant loss would be a big sign of trouble ahead for his candidacy, making a Clinton nomination appear all the more inevitable. Pundits have questioned whether it would be worth it for Sanders to carry on.

In the Democratic primary, 130 delegates will be up for grabs. Republicans will battle over 59 delegates. Polls close at 8 p.m.