Milwaukee: Republican Ted Cruz scored a decisive victory over Donald Trump in Wisconsin, giving him momentum at a crucial time and putting pressure on the front-running Trump to show he can absorb the shock and bounce back in upcoming primary states.
Cruz, a US senator from Texas, dominated the vote in Wisconsin on Tuesday, a sign that he is increasingly viewed as the main Trump alternative by those Republicans who cannot bring themselves to support the billionaire. His win increased the chances of a rare contested party convention in July.
"What an incredible victory tonight," Cruz said in his victory speech in Milwaukee, joined by Wisconsin Governor Scott Walker, a former rival who had endorsed him. "Tonight is a turning point. It is a rallying cry."
Cruz's victory injected fresh energy into what had been a flagging anti-Trump movement and showed that Trump has work to do to repair damage he brought to his campaign with remarks about abortion that hurt him with Republican women voters.
On the Democratic side, US Senator Bernie Sanders of Vermont easily defeated front-runner Hillary Clinton in Wisconsin, adding to Clinton's frustration that she has not been able to put away her rival and march to the Democratic presidential nomination.
But despite winning six of the last seven states, Sanders 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. Still, his victory was another sign that a sizable group of Democrats are not sold on the viability of Clinton's candidacy.
For Trump, the pressure is on to respond with some decisive victories in upcoming states to show he is still on the way to assembling the 1,237 delegates needed for the Republican presidential nomination.
Trump was heading to favourable turf in the Northeast and was already predicting victory in New York. It is now critical that he do well in these states.
"It's very important for Trump to bounce back strong. The sense of his inevitability is one of his strengths," said David Yepsen, director of the Paul Simon Public Policy Center at Southern Illinois University.
"If he looks weak, others will pile on and some may be tempted to leave him."
Trump, who had campaigned heavily in Wisconsin, responded to the defeat with a blistering attack on Cruz, saying he had been aided by Wisconsin conservative talk show radio hosts and millions of dollars in ads spent by an anti-Trump SuperPAC, or independent funding group.
"Ted Cruz is worse than a puppet - he is a Trojan horse, being used by the party bosses attempting to steal the nomination from Mr Trump," Trump's campaign said.
Cruz's victory was a new indication that the Republican race may be headed toward a contested convention, given the increasingly difficult path for either Trump or Cruz to put together enough delegates needed to win the nomination outright.
If no candidate reaches 1,237, the Republicans' choice will be picked when delegates gather for the party's national convention in Cleveland in July.
Trump, perhaps sensing a defeat in Wisconsin, retreated to his home base in New York on Tuesday and planned an event on Long Island on Wednesday, followed by a West Coast swing, including a news conference in the Los Angeles area, on Friday.
His campaign is taking steps to reflect greater seriousness of purpose, with plans for Trump to deliver a series of policy speeches intended to give him a more presidential image.
"The new Trump campaign needs to properly adjust to this new world order," said Republican strategist Scott Reed. "And Trump the candidate needs to embrace message discipline. Every delegate counts."
From Wisconsin, Cruz will spend two days of campaigning in New York, proof that he is not going to cede Trump's home state to his rival. He will visit a charter school in New York City on Wednesday and hold a rally at a Christian academy in upstate New York on Thursday.
"Our strategy is what it's always been," said Cruz spokeswoman Alice Stewart. "To win you have to have majority plus one. That's what we're focused on. No matter what state we're in, our objective is to acquire delegates to achieve 1,237."
The Wisconsin primary followed a difficult week for Trump, who was forced to backtrack after saying women who had 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, as his recent gains mark the first time since November that a rival has threatened Trump's standing at the head of the Republican pack.