Ted Cruz and Bernie Sanders each won his Wisconsin primary by 13 points.
More than two months after voting began in Iowa, front-runners Donald Trump and Hillary Clinton crashed into a cheddar wall.
But Clinton remains almost certain to ultimately secure the Democratic nomination, while The Donald now finds himself facing an emboldened opposition and an increasingly likely contested convention.
Six key indicators from the preliminary network exit polls of Republican primary voters:
• Thirty-eight per cent said they would feel "scared" if Trump became president, and 20 per cent said they would be "concerned."
• More than 1 in 3 said they would not support Trump in a hypothetical general election.
• More than half said Trump has run "the most unfair campaign."
• Nearly half were looking for a president with experience in politics.
• Cruz edged Trump among those who had only a high school education and among those in the lowest income brackets. That's been Trump's base.
• More than half of Wisconsin primary voters said they were "dissatisfied" with government, compared with one-third who were "angry." Trump consistently does better among those who are angriest.
The coverage Wednesday was brutal.
Trump has only himself to blame for recent stumbles, wrote The Washington Post's Karen Tumulty, Jose A. DelReal and Robert Costa: "The strength and the weakness of his unconventional campaign has been Trump himself," they wrote.
"He has functioned in many ways as his own chief strategist, political consultant, policy czar and communications guru. That is the same way he built his fortune, said campaign spokeswoman Hope Hicks.
'In real estate, he builds a building. . . . His name is on the building, and it's his product,' Hicks said. . . . The limitations of that approach have become more apparent as the campaign has been spread more thinly and as the field of competitors has narrowed."
Eliana Johnson, the National Review's Washington bureau chief, wrote, "More than a loss for Trump, it was a galvanising moment for the forces aligned against him."
ABC political director Rick Klein: "A hard truth is emerging for the GOP: at a moment where the frontrunner should be getting stronger, Trump is getting weaker."
The Trump campaign emailed reporters a defiant statement: "Donald J. Trump withstood the onslaught of the establishment yet again. Lyin' Ted Cruz had the Governor of Wisconsin, many conservative talk radio show hosts, and the entire party apparatus behind him. . . . Ted Cruz is worse than a puppet - he is a Trojan horse, being used by the party bosses attempting to steal the nomination from Mr. Trump."
Cruz showed he can unite the Stop Trump movement: "Give the Texas senator credit: When he needs to win, he wins," wrote The Fix's Chris Cillizza. "If Cruz had lost Wisconsin, pressure would have ramped up on him to bow to the political reality of Trump as the nominee. Now, he can make the case that Wisconsin fundamentally changed the trajectory of the race."
So what's next?
The battle enters a new phase: "The Republican race is about to become granular," The Post's Dan Balz explains. "The coming battles will be waged in targeted congressional districts where Trump shows weakness regardless of his statewide appeal, in hand-to-hand competition at state party conventions where the delegates are being selected, and ultimately in a battle for the hearts and minds of the men and women who will go to Cleveland, bound or unbound on the first ballot but free agents after that."
Trump plans to give a series of policy speeches to show he's "presidential," addressing topics such as how he would strengthen the nation's military, how he would reform education and the criteria by which he would select Supreme Court justices.
New York votes in 12 days. Trump should win the state, and polls show him over 50 per cent. "But New York's rules are less than ideal for Trump at a time when he needs to sweep up as many delegates as possible," Balz explains.
"For starters, the state's 95 delegates will be awarded proportionally, rather than on a winner-take-all basis. Beyond that, 81 of those delegates are distributed on the basis of results in the state's 27 congressional districts."
Quite a cast of characters is helping Trump in the Empire State: Carl Paladino, the 2010 GOP gubernatorial nominee, is one of Trump's co-chairs in the state. And former congressman John Sweeney, R-New York, has been hired by the campaign to help out for the next two weeks, per Maggie Haberman of the New York Times. Sweeney was arrested twice on drunken-driving charges after losing to now-Sen. Kirsten Gillibrand (D) in 2006.
Cruz now must capitalize on his victory by winning over the GOP establishment and raising money.
But there is still a lot of bad blood. Rep. Peter T. King, R-New York, on CNN, declined to say who he'll vote for.
"But I can tell you one thing: I would never vote for Ted Cruz," said King, who backed Marco Rubio until he dropped out. The Hill notes that King told Cruz to "go back under a rock" when the candidate questioned Trump's "New York values" in January.