Ted Cruz wins Wisconsin in blow to Donald Trump
Republican presidential candidate Ted Cruz has decisively won the Wisconsin primary, complicating front-runner Donald Trump's path to the nomination.
In the Democratic race, Bernie Sanders scored a strong victory over Hillary Clinton in the Midwestern state.
Mr Trump leads the race, but could fall short of the number of delegates needed to secure the Republican nomination.
That would mean a contested convention where voting for candidates starts again from scratch.
How does a contested convention work?
Mr Trump said on Tuesday he would prevail despite the loss and took aim at his main rival.
"Ted Cruz is worse than a puppet – he is a Trojan horse, being used by the party bosses attempting to steal the nomination," the Trump campaign said in a statement.
Party leaders are concerned that Mr Trump would be a weak candidate in the general election and could harm other Republicans lawmakers on the ballot. Polls show that the real estate tycoon is extremely unpopular among key voting blocs including women, Latinos and young people.
Mr Sanders won nearly every county in the state except Milwaukee, but as delegates are awarded proportionally he will not gain a significant advantage over Mrs Clinton. Of the 86 Wisconsin delegates, Mr Sanders is on course for at least 44, but Mrs Clinton will have at least 28.
Mrs Clinton still holds a sizeable lead and most analysts say she will eventually become the Democratic nominee despite her recent losses.
While Tuesday's loss was a setback for Mr Trump, his campaign has time to rebound. The campaign now moves to large north-eastern states, where polls show Mr Trump holds significant leads.
Mr Trump's loss in Wisconsin comes after a rocky week for the campaign, particularly with female voters. The New York businessman repeatedly struggled to articulate his position on abortion. At one point, he called for women to be punished for having abortions, then quickly changed his mind.
His campaign manager Corey Lewandowski was also arrested, accused of manhandling a female journalist.
Analysis: Jon Sopel, BBC News North America Editor
"Tonight is a turning point, it is a rallying cry to the people of America," Mr Cruz told supporters in Milwaukee on Tuesday. "We are winning because we are uniting the Republican Party."
Mr Cruz is unlikely to earn enough delegates to win the nomination outright, but Republican Party leaders have rallied around the Texas senator in hopes of wounding Mr Trump.
For nine months now it has seemed that Donald Trump could say and do whatever he liked without there being consequences.
But then he took on women. Well to be strictly accurate he had taken on women before, with seemingly no ill effect. But then a few things came together in quick succession.
The insulting photo of Heidi Cruz, the suggestion that women should be punished for having an abortion if it is outlawed, Mr Trump standing up for his campaign manager when he is charged with assaulting a female journalist, crystallised into his poll standings falling, too.
And so Wisconsin is lost. And Mr Trump has shown he is mortal.
US media on the Cruz-Trump duel
The New York Times said that in winning so convincingly, Mr Cruz had "showed he was capable of appealing to more than just the hard-line and religious conservative Republicans", but that capturing moderate Republicans and consolidating the anti-Trump vote would be a "daunting task".
The Washington Post said that the Wisconsin primary had taken the Republicans into a "new and critical phase in their volatile nomination battle" with a contested convention "more probable".
USA Today said Mr Cruz now "represents something very powerful for many GOP voters: their last best hope for denying Trump the nomination".
The Los Angeles Times said Wisconsin should have been "tailored" to Mr Trump's advantage but the frontrunner made a "series of tactical miscues", including insulting the state's popular governor, Scott Walker, and insulting Mr Cruz's wife, Heidi.
Analysis site Vox pointed out that even if Mr Cruz takes every delegate in Wisconsin, he will still trail Mr Trump by about 240 delegates.