Wisconsin’s presidential primary is shaping up as a wide-open, high-turnout, stand-alone battleground that poses a late challenge to both front-runners, Republican Donald Trump and Democrat Hillary Clinton.

For Trump, the state should be a demographic match made in heaven, rich in the kinds of rural and blue-collar voters who have flocked to his campaign. But polls show Trump with high negatives among GOP voters in the very region that typically decides Republican primaries, metropolitan Milwaukee. His soft polling numbers have encouraged “Stop Trump” activists and Trump’s two remaining GOP rivals to make a stand here in the April 5 primary.

“They’re all playing in Wisconsin — hard,” said former Gov. Tommy Thompson, a John Kasich supporter. “Wisconsin is crucially and critically important — to see if ‘the Donald’ is going to have the momentum, is [Ted] Cruz really going to be able to compete with him and, in the Kasich campaign, do we have a path forward?”

Trump said Monday that he hoped to do well in Wisconsin, but conceded, “There will be some [states] we don’t” win as he tries to rack up enough delegates to avoid a contested convention.

On the Democratic side, Clinton’s 17-point loss to Barack Obama in the state’s 2008 primary marked a low point in her last run for president. Wisconsin’s mostly white electorate, open primary, progressive streak and history of turning out young voters make it friendlier turf for rival Bernie Sanders.

Sanders strategists twice referred to Wisconsin as a “big showdown.” Campaign manager Jeff Weaver said it was one of the upcoming states “where we’re going to have to do very well.”

Clinton campaign spokesman Yianni Varonis said that “Hillary Clinton is committed to Wisconsin.” But the campaign hasn’t said whether Clinton will spend significant time in the state in the next two weeks.

Some very big unknowns loom over Wisconsin.

One is turnout. Wisconsin will be the 34th state to vote, and Trump and Clinton are both in command of their races. Will that dampen turnout, or will voters here jump at the chance to have their say?

If the very bullish turnout projection by state election officials pans out — 40 percent of voting-age adults — all bets are off.

Then there’s the state’s open primary. Since Wisconsinites don’t register by party, there are no barriers in either contest to independents or crossover voters from the other party.

One more wild card: Wisconsin is the only primary on either side for the next two weeks (three states have Democratic caucuses Saturday). It’s the first time since the opening rounds in Iowa, New Hampshire and South Carolina that candidates will be fighting over a single state for any length of time.