Controversial Republican hopeful Donald Trump has suffered a heavy blow after rival Ted Cruz won the Wisconsin primary overwhelmingly.
While Trump remains the frontrunner, the development means the Republican establishment will be more likely to be able to block his nomination.
If Trump does not get half of the required delegates before the Republican national convention, there will be a contested convention, where party leaders can choose the nominee. That means Cruz could be picked, if he could convince the convention that he should be the Republican runner.
"Tonight is a turning point, it is a rallying cry to the people of America," Cruz said. "We are winning because we are uniting the Republican Party."
But Trump hit out at the victory. His campaign said: "Ted Cruz is worse than a puppet – he is a Trojan horse, being used by the party bosses attempting to steal the nomination."
The news comes amid concern from the Republican establishment that Trump would be a weak candidate to take on the Democrats in a head-to-head, as he is unpopular with key voting groups such as women and Latinos.
Odds of Trump winning the nomination have shifted from 4/6 to 4/5 with Betway on the news, while Cruz was cut from 9/4 to 2/1. And William Hill lengthened his chances on becoming president, pushing him from 3/1 to 5/1.
Nationally, Cruz has 35.2 per cent support among Republicans, against Trump's 39.5 per cent, according to Reuters/Ipsos polling data taken from 1-5 April. This puts the two within the poll's credibility interval of 4.8 percentage points.
Trump unveiled more foreign policy plans yesterday, and proposed using anti-terrorism laws to cut off remittances from Mexican people living in the US illegally unless Mexico makes a one-time payment of up to $10bn (£7bn) towards the wall he wants to build along the US border.
On the Democrat side of the race, senator Bernie Sanders also won convincingly over favourite Hillary Clinton.