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 Republican race, but there are concerns that he could fall short of the number of delegates needed to secure the party's nomination.
Mr Trump's rivals have pinned their hopes on a contested convention.
At a contested convention, party leaders, not voters, would choose the nominee.
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.