They're voting in Wisconsin today and for two candidates, it's a question of keeping their campaigns viable. Victories for Ted Cruz and Bernie Sanders won't materially change the race. Their respective partys' front runners - Donald Trump and Hillary Clinton - will maintain large leads.

But a loss by either candidate would weaken their argument for continuing with the race. For Sanders especially, who has won 5 primaries and caucuses in a row, it's vital to keep the Democratic party questioning whether they really want to nominate such a weak candidate in Hillary Clinton.

And given Clinton's huge lead in delegates, some believe Sanders must not only win, but win big.

The Hill:

But problem for Sanders, they say, is that it is not enough for him simply to eke out a tight victory in Wisconsin. If Sanders cannot win big in Wisconsin – a state that naturally favors him given it has an overwhelmingly white electorate and a proud progressive history – he might as well go home, said veteran Democratic strategist Joe Trippi. 

“It’s gotta be 10 points,” Trippi said. “If he can’t win by 10 points in Wisconsin then where the hell is he going to make up the delegates?” 

Sanders trails Clinton by nearly 300 pledged delegates, and because Democrats award delegates proportionally, Sanders needs to not only win states from here on in, but to beat Clinton resoundingly to close the delegate gap. 

In a recent interview with The Hill, Sanders’s top strategist Tad Devine said the campaign had shifted to a strategy of winning states and proving that the Vermont Senator has appeal across the nation. Devine said the campaign is determined to press on and a Wisconsin win will provide another data point to tell supporters that Sanders is on a roll. 

Trippi said some voters might fall for that line and keep sending checks – Sanders raised an astonishing $44 million last month despite trailing Clinton badly in delegates – but while the strategy might keep Sanders in the race until the convention, he has no path to the nomination unless he crushes Clinton in Wisconsin and then beats her in the New York primary on April 19. 

Although Sanders has spent more time in Wisconsin over the last week than Clinton, there have been some concerning signs in recent days for Sanders’s campaign, said Thad Nation, a Badger State Democratic strategist who is unaffiliated in the 2016 Democratic primaries. 

A rally in at the University of Wisconsin-Madison on Sunday night had an estimated 5,000 people attend but the Kohl Center venue had a capacity of 17,000 and expectations were that Sanders would fill it, particularly given the young demographic he attracts. 

Then, on Monday, Nation observed that the Sanders campaign had moved its Milwaukee rally at the last minute to a much smaller venue – a "very significant" change, he said.  

Ted Cruz has his own challenges. He must break the Trump mystique and win a state with a large number of blue collar white voters - a demographic Trump has dominated elsewhere. If Cruz can demonstrate appeal across the board in Wisconsin, it will fuel his main argument; Trump is unelectable.

Trump hasn't helped himself this week:

Trump, who has steadily maintained he will pull off an upset victory in Wisconsin, has polled relatively stable at 30 percentage points but his popularity has hit an apparent ceiling, Franklin said, for three reasons.  

Wisconsin’s Republican elites and elected officials have coalesced around Cruz, and the state’s right-wing talk radio hosts have been pounding Trump since September for not being a true conservative. The conservative talk radio infrastructure in Wisconsin is among the most powerful and ideologically developed in the country and it is squarely against Trump. 

The third reason, Franklin said, is that Trump appears to have underestimated just how unifying and beloved a figure Gov. Scott Walker is in Wisconsin. Franklin said he can’t figure out why Trump thought it was a good idea to come to the Badger State and attack the popular sitting governor. 

“Going after a guy with an 80 percent approval rating is kind of puzzling,” Franklin told The Hill on Monday. 

And it's an open question whether Trump can put last week's unfrorced errors behind him:

In recent days, Trump has advocated for the spread of nuclear weapons to allies, reversing decades of U.S. policy; he stumbled over the role of the federal government, naming education and health care as key areas of needed involvement, before quickly walking his comments back. And, most notably, he said women who have abortions should be punished if that procedure is made illegal. He quickly reversed that position, saying he misspoke, but not before castigating both sides on the abortion rights issue.

On Friday, in an interview with CBS, Trump dug himself an even deeper hole in the eyes of many Republicans, arguing that abortion should remain legal.

Trump did no damage to his base who will vote for him anyway. But if he expects to win, he needs to attract persuadables, including independents and more educated whites. Polls in Wisconsin are unreliable given the fast moving series of events that occurred late last week. But if Cruz wins big, it might give Cruz even more momentum in his hunt for delegates - something that is looking more and more crucial as Trump's chances of winning the nomination outright recede.