Wisconsin's highly engaged voters routinely post some of the highest turnout numbers in presidential elections. On Tuesday, they will play an important role in both major-party nominating contests. Here are some of the things we will be watching.
Look to the north
Donald J. Trump hit a rough patch, to put it mildly, in the run-up to Wisconsin, where he trails Senator Ted Cruz of Texas in statewide polls. A victory for Mr. Cruz, who has mostly won in caucus states up till now, would demonstrate that his appeal is growing, and would be an indicator of lasting damage to Mr. Trump's candidacy.
"Trump needs to regain some momentum after a disastrous week," said Brian Walsh, a Republican strategist unaffiliated with the candidates. "Cruz needs to demonstrate that he can win a big-state primary besides Texas."
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.
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.
Polls have shown Mr. Trump doing poorly in Wisconsin for months, and he lost in neighboring Iowa and Minnesota. Wisconsin voters tend to attend church frequently and to be highly educated, both characteristics that often hurt Mr. Trump, exit polls show. And after several closely contested elections, including the failed recall of Gov. Scott Walker in 2012, an already sophisticated electorate is even more engaged — something that might not bode well for Mr. Trump given his weakness in polls about possible general-election matchups.
If Mr. Cruz wins easily — say, by more than 10 percentage points — few will argue that it was anything but a good showing for him. But a closer contest could be followed by a debate about whether Mr. Trump had really lost ground or merely hit a speed bump on his road to the nomination.
If Sanders wins, how much ground does he gain?
Wisconsin is an overwhelmingly white state with a progressive political tradition, so it was no surprise that Senator Bernie Sanders held a modest lead over Hillary Clinton in Democratic polls leading up to the primary.
But Mr. Sanders's overall deficit — he trails Mrs. Clinton by about 230 delegates — is becoming so large that winning only modest victories puts the Democratic nomination farther out of his reach. He needs to win around 57 percent of the outstanding vote to win a majority of pledged delegates. If he falls short of that bar in Wisconsin, his target in later states will be ratcheted up even higher.
Mr. Sanders might still pull off a big win. Barack Obama won the state by 17 points in 2008, after all. But a big win for Mr. Sanders would not necessarily put him on track to win the nomination. Even an overwhelming victory for the senator might only narrow Mrs. Clinton's lead by 20 delegates.
How uphill is Clinton's climb in the Midwest?
Mrs. Clinton may be on track to win the nomination, but the upper Midwest has been a mixed bag for her. She narrowly won the Iowa caucuses, but lost big in the Minnesota caucuses. And Michigan — where Mr. Sanders won white voters by a 14-point margin, hammering Mrs. Clinton on trade — is still the only big primary state she has lost.
A poor result in Wisconsin could be a sign of Mrs. Clinton's deeper challenges ahead in a region that is essential to a Democratic victory in November.