WASHINGTON (AFP) - Wisconsin takes centrestage on Tuesday (April 5), when the mid-western state votes in the US presidential primary race, with Republican front runner Donald Trump and top Democrat Hillary Clinton seen as underdogs against their never-give-up rivals.
The potentially pivotal contest is the first after a 10-day lull in the process that determines the parties' nominees for the November general election, with Mr Trump's main rival Senator Ted Cruz eyeing the Badger State as a crucial firewall against the celebrity billionaire's march to an outright nomination win.
But should Mr Trump, who is riding a wave of anti-establishment anger, manage to snatch a surprise victory there, he could suffocate Mr Cruz's campaign.
"If we do well here, folks, it's over," he claimed at a campaign stop on Monday in the town of La Crosse.
Like Mr Trump, Mrs Clinton risks losing Wisconsin, where she faces a surging Mr Bernie Sanders, who has won five of the last six contests.
But April could ultimately prove to be a sunny month for the former secretary of state, who leads Mr Sanders by double digits in New York, which votes on April 19, and Pennsylvania, which casts ballots a week later.
Mr Trump, the 69-year-old real estate mogul from New York, also leads handily in those states.
The birthplace of the Republican Party, Wisconsin is seen as Ground Zero for the anti-Trump movement.
Halting him there would bolster Mr Cruz, the 45-year-old conservative senator from Texas.
For Mr Cruz, "it's a very important win. For Trump, it's not a critical loss", University of Iowa professor Timothy Hagle told AFP.
However, Mr Trump has been in damage control this past week.
Although his campaign had recently seemed bulletproof, his latest controversial statements - on abortion, Mr Cruz's wife and a journalist who said she was roughed up by Mr Trump's campaign manager - have further alienated women voters, polls indicate.
With polls showing Mr Cruz leading in Wisconsin, Mr Trump's wife Melania - a former model of Slovenian origin - joined him on Monday at the Milwaukee Theatre for a rare campaign trail speech, part of a likely effort to boost flagging support among women.
"He's fair," insisted Mrs Trump, 45.
"As you may know by now, when you attack him, he will punch back 10 times harder," she said, but added: "No matter who you are, man or a woman, he treats everyone equal."
Mr Trump doubled down on some of his recent controversial recent - that the United States should consider leaving Nato and that Japan should be responsible for its own nuclear defence.
He also offered a fresh economic doomsday prediction.
"We're going to go into a massive recession," he warned. "But I also say, if I'm president that's not going to happen."
The winner of Tuesday's Republican primary will take most of the 42 delegates on offer.
If Mr Cruz wins, he will claim the victory as a turning point in the race, although he will have to struggle to overcome his delegate deficit.
Currently, Mr Trump has 739 delegates, Mr Cruz, 466 and Ohio Governor John Kasich, 145, according to CNN's tally. A candidate needs 1,237 delegates to win the Republican nomination outright.
Mr Trump on Monday called for Mr Kasich to "get the hell out" of the race. But Mr Kasich has refused, insisting he would be the logical mainstream choice in the event of a contested convention in July.
Mr Trump predicted he could sew up the nomination before the confab in Cleveland.
"I think we're going to get there on the 1,237 if you want to know the truth," he said.
Mr Cruz was also positive, telling Wisconsin voters that "our campaign still has a clear path" to crossing the delegate threshold before the convention.
Some experts contested that assertion.
"There's no chance he can get to 1,237," veteran election watcher Larry Sabato of the University of Virginia said of Mr Cruz.
"It's simply a matter of whether Trump gets to 1,237. If he does, it's over," he added.
"If he doesn't, then we go on to other ballots (at the convention) and anything can happen."
Mrs Clinton meanwhile spent Sunday and Monday morning in New York, returning only Monday night to Wisconsin, a possible sign of how she sees her chances there.
"Between you and me, I don't want to get Hillary Clinton more nervous than she already is," Mr Sanders quipped to voters in Janesville.
"She's already under a lot of pressure.
"So don't tell her this, but I think we win here," he said, adding that "we win in New York state, we're on our way to the White House",
Grassroots enthusiasm for the self-proclaimed democratic socialist remains high.
Mr Sanders out-fundraised Mrs Clinton in March, pulling in a stunning US$44 million (S$59.6 million) in donations against her US$29.5 million.
But he faces an uphill struggle. In order to prevail, he would need to win at least 60 per cent of all remaining delegates.
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