MILWAUKEE - As polls opened across Wisconsin on Tuesday, the front-runners in the presidential race - Donald Trump and Hillary Clinton - braced for results that could deliver embarrassing setbacks and further unsettle contests on both sides.
A loss for Trump in the GOP primary could reset a Republican contest that has been dominated by his outsider candidacy and outsized media presence. State polls show Trump in a tight race with Sen. Ted Cruz of Texas, prompting speculation about whether Wisconsin could expose cracks in the billionaire's high-flying nomination run.
Those same polls show Clinton in a similarly tight race with Vermont Sen. Bernie Sanders, whose populist, anti-Wall Street message has transformed what was expected to be an easy nomination race for Clinton.
Winning Wisconsin, where polls will close at 9 p.m. EDT, would give Sanders a fresh dose of momentum - and perhaps new credibility for his claim that he has a chance to catch Clinton in the delegate count and win the Democratic nomination.
Both Trump and Clinton maintain they can still win in Wisconsin.
But on the Democratic side, Sanders's unexpected staying power has unnerved some of Clinton's supporters. In a memo sent out to backers Monday evening, Clinton campaign manager Robby Mook sounded aggravated.
"Hillary Clinton has built a nearly insurmountable lead among both delegates and actual voters," he wrote. "Contrary to the claims of the Sanders campaign, in measure after measure, Clinton has shown the broadest support of any candidate currently running for president. We know that the misleading spin will continue, but we wanted you to know the facts about the real state of the Democratic primary."
Mook said Sanders would have to win by overwhelming margins the four biggest delegate prizes left, including her home state of New York, to erase Clinton's big lead. He did not mention Wisconsin.
Trump stumbled badly during the run-up to Wisconsin with a gaffe over abortion in which he stated, and then retracted, that women who seek abortions should be punished. He also took a hit over comments on foreign policy that led President Obama to say that the businessman knows little about the world.
Partly as a result, the Wisconsin race has emerged as a key moment in the Republican nomination, exposing weaknesses for Trump in an industrial state with a large working class - demographics that have favored him in other states.
A Marquette Law School poll released last week showed Cruz surging to 40 percent support among likely GOP voters, up from 19 percent in a February poll. Trump's support remained steady at 30 percent.
Trump has refused to concede that polls showing him trailing Cruz are accurate even while acknowledging that a string of controversies may have cost him votes. And he has dismissed speculation about a possible break in his momentum by pointing to his strengths in New York - his home state, which holds its primary on April 19 - and in the string of Eastern states that vote later this month.
"I've been given the last rites how many times, like 10? Every week, it's the end of Trump," the billionaire said during a campaign rally in Superior, Wis. "Then they walk in, 'Sir, I don't know what happened but your poll numbers just went through the roof.'"
Anti-Trump Republicans, who have poured millions of dollars into attack ads around the country, are hopeful that a loss in Wisconsin will signal a break in the momentum that has kept Trump steadily rising in the polls.
A loss in Wisconsin, they believe, will increase the likelihood of a contested Republican convention in July - a strategy that rests on keeping him from crossing the requisite 1,237-delegate threshold he needs to clinch the nomination outright.
Speaking of "Fox and Friends" Tuesday former New York mayor Rudy Giuliani - who has not made an endorsement in the race - said Wisconsin Republican Gov. Scott Walker's support for Cruz has given him a critical boost.
"Walker may have one of the better operations in the country, because of the way it's been tested," Giuliani said, referring to the 2012 recall challenge and other elections the governor has survived. "Absent the Walker machine, Trump wins Wisconsin."
In a private document circulated over the weekend and obtained by The Washington Post, Trump campaign senior adviser Barry Bennett revealed the mounting frustrations among the billionaire's top aides as they closed what had been a tumultuous week.
Entitled "Digging through the Bull [expletive]," Bennett's memo urged Trump campaign manager Corey Lewandowski - who was charged with battery last week for allegedly yanking a reporter - and others to ignore critics who have questioned whether Trump's campaign for the Republican presidential nomination has waned.
"America is sick of them. Their idiotic attacks just remind voters why they hate the Washington Establishment," Bennett wrote, citing tracking poll data favorable to Trump.
"Donald Trump 1," Bennett declared, as if he was scoring the past week. "Washington Establishment/Media 0."
Clinton campaigned fewer days and before smaller crowds in Wisconsin than Sanders, and turned much of her attention ahead to the larger stakes in New York. She did not mention the Wisconsin race during a rally Monday in Manhattan to cheer the state's approval of a $15 minimum wage.
On the eve of the primary, Sanders touted his long-standing support for labor unions and his opposition to a series of "disastrous trade deals," setting a contrast with Clinton that he's pressed in other industrial, midwestern states.
The senator from Vermont began his day in Janesville, where General Motors shuttered a manufacturing plant in 2008 and moved operations to Mexico, costing the community 2,800 jobs, Sanders said.
"I am not a candidate who goes to the unions, goes to workers, then leaves and goes to a fundraiser on Wall Street," he said, taking a jab at Clinton and her ties to the financial sector.
Clinton has also been critical of Sanders's posture on trade while campaigning in Wisconsin.
At a Democratic dinner in Milwaukee on Saturday, she said Sanders seems to pride himself on having "opposed all trade deals, all the time."
"But I don't think that's right," because when "done right" trade arrangements can benefit American workers, Clinton said.
Aides to Clinton, who is spending day in New York City, has been telegraphing a potential loss in Wisconsin for months. She is scheduled to appear on ABC's morning program "The View" and hold a "Women For Hillary" town hall-style event in Brooklyn in the afternoon.
Her campaign announced no plans for an election-night party. She planned an evening fund-raising party in New York, although the campaign did not provide details ahead of time.
Sanders has made political hay with Clinton's fund-raising schedule, frequently noting that his donations are almost all raised online, in small amounts, while she relies heavily on big checks from wealthy donors.
The third Republican still in the race, Ohio Gov. John Kasich, is far behind his two rivals in recent Wisconsin polls. He spent Monday campaigning in New York in advance of that state's primary in two weeks.
Forty-two delegates are at stake for Republicans, while 96 delegates will be awarded in the Democratic Party. Since delegates are assigned proportionally on the Democratic side, Sanders will have a difficult time slicing into Clinton's overall lead if he does not win by an overwhelming margin.
The Washington Post's John Wagner in Janesville, Wis., and Robert Costa and Abby Phillip in Washington contributed to this report.