Republican Ted Cruz won the Wisconsin presidential primary on Tuesday, dealing a blow to front-runner Donald Trump's hopes of amassing the delegates needed for the party's nomination ahead of the July convention and boosting the chances of a rare contested convention.
Cruz's win was a breakthrough for Republican Party forces battling to block the controversial New York billionaire, and it raised the prospect of a prolonged nominating fight that could last to the July convention.
Democratic presidential contender Bernie Sanders also won in Wisconsin, gaining momentum in his fight against front-runner Hillary Clinton and trimming her commanding lead in delegates.
Trump had 737 convention delegates to Cruz's 481 heading into the vote, leaving him 500 delegates short of the 1,237 needed to become the party's nominee in the November 8 election. Cruz and Ohio Governor John Kasich, the other remaining Republican contender, hope to stop Trump short of a first-ballot victory and trigger a contested convention.
Cruz, a conservative US senator from Texas, was aided in Wisconsin by the backing of Republican Governor Scott Walker, who had dropped his own presidential bid in September. Party establishment figures, worried that Trump will lead Republicans to a broad defeat in November, have banded together to try to stop him.
The Wisconsin primary followed a difficult week for Trump, who was forced to backtrack after saying women who have abortions should face punishment if the procedure is outlawed, and who voiced support for his campaign manager after he was charged with misdemeanor assault for grabbing a reporter.
A new Reuters/Ipsos poll on Tuesday showed Cruz about even with Trump nationally, with Cruz's recent gains the first time since November that a Trump rivals has threatened his standing at the head of the Republican pack.
The poll, taken April 1 to 5, showed Cruz winning the support of 35 per cent of Republicans to Trump's 39 per cent, within the credibility interval for the survey of 568 Republicans. Cruz and Trump were also briefly about even early last week.
As recently as a month ago, when Senator Marco Rubio was also still a candidate, Cruz trailed Trump by about 20 points.
In the Democratic race, the win for Sanders, a US senator from Vermont, is his sixth in the last seven presidential nominating contests, but he still faces a difficult task to overtake Clinton as the presidential nominating race moves to New York on April 19 and to five other Eastern states on April 26.