Donald Trump and Ted Cruz have made spirited final pitches to Wisconsin voters who will cast ballots in a Republican primary that both consider a key step in the race for president.
After Tuesday, there's a two-week lull before the next important voting, in New York.
Trump is facing pressure on multiple fronts following a difficult week marked by his controversial comments, reversals and rare moments of contrition.
Those included re-tweeting an unflattering photo of Cruz's wife and a series of contradictory comments on abortion that managed to draw condemnation from both abortion rights activists and opponents.
Trump is the only Republican with a realistic path to clinching the nomination ahead of the Republican convention, but a big loss in Wisconsin would greatly reduce the billionaire real estate mogul's chances of reaching the needed 1237 delegates before then.
A big win for Trump would give him more room for error down the stretch.
On the Democratic side, polls show Vermont senator Bernie Sanders leading Hillary Clinton, but not by a wide enough margin to make a significant reduction in her overall delegate lead.
Sanders hopes that a win in Wisconsin will build more momentum against Clinton heading into the New York primary. Sanders has won five of the past six states to hold contests.
In Wisconsin, Trump has been battered by negative ads. The state's top Republican advertiser has been Our Principles PAC, which pumped almost $US1.3 million ($A1.71 million) into anti-Trump ads.
The Club for Growth, which has endorsed Cruz, is spending $US800,000 on ads that promote voting for the Texas senator - not Ohio Governor John Kasich - as the best way to ensure a Trump defeat.
Also, the state's Republican establishment, including Governor Scott Walker and some of its most influential conservative talk radio hosts, have lined up to support Cruz. Polls show Cruz, an uncompromising conservative, with a lead in Wisconsin.
On the campaign trail, both he and Cruz were optimistic.
"I really believe tomorrow we're going to have a very, very big victory," the billionaire businessman said at a stop in La Crosse. Later, in a frigid hangar at the Richard I. Bong airport in Superior, Trump joked about predictions of his demise from pundits.
"You know how many times I've been given the end? I've been given the last rites, how many times? Like 10? Every week, it's the end of Trump," he said.
Cruz's confidence was growing, too. He predicted a "terrific victory" during the taping of a town hall in Madison that was broadcast on Monday night on Fox News.
Cruz also discounted any possibility of someone other than Trump or him winning the nomination. Some Republican party leaders, concerned that both Trump and Cruz might fare poorly in the November election, have expressed hope that a contested convention might nominate a non-candidate like House Speaker Paul Ryan.
"This fevered pipe dream of Washington that at the convention they will parachute in some white knight who will save the Washington establishment, it is nothing less than a pipe dream," Cruz told reporters. "It ain't going to happen. If it did, the people would quite rightly revolt."
The polls close on Tuesday at 8pm local time (1100 AEST Wednesday).
The Democratic contest has taken a more contentious turn in recent weeks, as the candidates turn their focus to what's expected to be a drag-out fight ahead of the New York primary.
On Monday, Sanders campaigned in Wisconsin, while Clinton was in New York, which is a must-win state for her.
Trump and Sanders ended their days with duelling rallies in Milwaukee. But both drew smaller-than-usual crowds on the frigid night, with hundreds of empty seats at the theatre where Trump spoke and a less-than-capacity crowd at the Wisconsin Center where Sanders was holding his final rally nearby.
