WASHINGTON - Voters in the Midwestern state of Wisconsin cast their ballots Tuesday in the US presidential primaries, with Donald Trump and Hillary Clinton cast as underdogs hoping to pull off surprise come-from-behind wins.
The stakes are highest for Trump, whose long ride as the Republican frontrunner has run into severe turbulence over flip-flops on abortion and other campaign gaffes.
As polls opened at 7:00 am (1200 GMT), both the Republican and Democratic races appeared to have narrowed considerably.
A poll average by the Real Clear Politics website showed Texas Senator Ted Cruz leading Trump 39.2 percent to 34.5 percent in the Republican contest, with Ohio Governor John Kasich trailing at 20 percent.
Vermont Senator Bernie Sanders was ahead of Clinton by a nose, 47.9 to 45.3 percent.
Cruz is eyeing the Badger State as a crucial firewall against the celebrity billionaire's march to an outright nomination victory.
But should Trump manage to snatch a surprise victory there, he could snuff out Cruz's campaign.
"If we do well here, folks, it's over," Trump claimed at a campaign stop Monday in the town of La Crosse.
Like Trump, Clinton risks losing Wisconsin, where she faces a surging Sanders who has won five of the last six contests.
But she can look forward with some confidence to the next races. She leads Sanders by double digits in New York, which votes April 19, and Pennsylvania, which casts ballots a week later.
Trump, the 69-year-old real estate mogul from New York, also leads handily in those states.
Wisconsin, the birthplace of the Republican Party, is seen as Ground Zero for the anti-Trump movement.
Halting him there would bolster Cruz, the 45-year-old conservative senator.
For Cruz, "it's a very important win. For Trump, it's not a critical loss," University of Iowa professor Timothy Hagle told AFP.
However, Trump has been in damage control this past week.
Although his campaign had recently seemed bulletproof, his latest controversial statements - on abortion, Cruz's wife and a journalist who said she was roughed up by Trump's campaign manager - have further alienated women voters, polls indicate.
Cruz tried to cash in on Trump's recent campaign gaffes that seem to be alienating women voters.
"He seems to have a problem with strong women," Cruz said on a Town Hall edition of Fox News Channel's "The Kelly File." With polls showing Cruz leading in Wisconsin, Trump's wife Melania - a former model of Slovenian origin - joined him Monday at the Milwaukee Theater for a rare campaign trail speech, part of a likely effort to boost flagging support among women.
"He's fair," insisted Melania Trump, 45. "No matter who you are, man or a woman, he treats everyone equal." Trump doubled down on some of his recent controversial comments - that the United States should consider leaving NATO and that Japan should be responsible for its own nuclear defense.
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.
Currently, Trump has 740 delegates, Cruz 474 and Kasich 145, according to CNN's tally. A candidate needs 1,237 delegates to win the Republican nomination outright.
Trump on Monday called for Kasich to "get the hell out" of the race. But Kasich has refused, insisting he would be the logical mainstream choice in the event of a contested convention in July.
Trump predicted he could sew up the nomination before the confab in Cleveland.
Cruz was also positive, telling Wisconsin voters that "our campaign still has a clear path" to crossing the delegate threshold before the convention.
"There's no chance he can get to 1,237," veteran election watcher Larry Sabato of the University of Virginia said of 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." Clinton 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," 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 added, "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.
But in order to prevail, he would need to win at least 60 percent of all remaining delegates.