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.
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.
Trump’s rivals have pinned their hopes on a contested convention.
At a contested convention, party leaders, not voters, would choose the nominee.
“Tonight is a turning point, it is a rallying cry to the people of America,” Cruz told supporters in Milwaukee on Tuesday. “We are winning because we are uniting the Republican Party.”
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 Trump.
Trump said on Tuesday he would prevail despite the loss and took aim at his 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 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, reports the BBC.
On the Democratic side, Wisconsin adds to a recent spate of wins by the Sanders campaign, giving the Vermont senator a boost before key races in New York and Pennsylvania.
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, Sanders is on course for at least 44, but Mrs Clinton will have at least 28.
Addressing supporters in Wyoming, which holds its primary on Saturday, Sanders stressed momentum was on his side and that his outsider candidacy could change the status quo.
“Real change never takes place from the top down; it always takes place from the bottom up,” Sanders told supporters.
Mrs Clinton still holds a sizeable lead and most analysts say she will eventually become the Democratic nominee despite her recent losses.
