But even if Mr. Trump lost the statewide vote count, and the 18 delegates that go with it, he could still mitigate the damage by winning in Wisconsin's Seventh and Eighth Congressional Districts, predominantly rural and heavily blue-collar districts covering much of the state's northern half.
Is Trump vulnerable in the suburbs?
Were Mr. Trump to win both districts, he could take six of Wisconsin's 42 delegates. That may not sound like many when 1,237 are needed to clinch the nomination, but every delegate is crucial if Mr. Trump is to avoid a floor fight at the Republican National Convention in July.
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If Mr. Trump has an opportunity to win delegates in sparsely populated parts of Wisconsin, he appears most vulnerable in the more crowded suburbs around Milwaukee, the state's Republican bulwark. This weakness might be what seals Mr. Trump's fate in the state.
But a more consequential question is whether Mr. Trump's difficulties among college-educated, middle-class voters in southeastern Wisconsin are confined to that region, where anti-Trump conservative talk radio hosts have considerable sway over Republican voters.
The primary moves next to a series of Northeastern and Mid-Atlantic States, including New York, Maryland and Connecticut, where the Republican electorate is largely suburban. If white-collar resistance to Mr. Trump shows itself there, too, several states he was expected to dominate could be up for grabs.
A Cruz blowout, or a Trump speed bump?
Mr. Trump's weak poll numbers in Wisconsin have raised questions about whether he might be losing ground after a series of unforced errors and unfavorable news coverage. But it is also possible that the state was always going to be challenging for him.