Ted Cruz has suspended his campaign for the Republican presidential nomination after losing the Indiana primary.
He told a crowd in Indianapolis: "I said I would continue on as long as there was a viable path to victory.
"Tonight I'm sorry to say it appears that path has been foreclosed.
"With a heavy heart, but with boundless optimism for the long-term future of our nation, we are suspending our campaign."
A campaign rally for Republican presidential hopeful Donald Trump has been postponed following widespread protests.
According to a Trump campaign statement, Trump met with law enforcement officials ahead of the rally in Chicago and decided it should be cancelled for safety reasons.
Thousands of people had gathered to protest against Donald Trump's candidacy.
US Republican presidential front-runner Donald Trump has been compared to Adolf Hitler by two former Mexican presidents.
Felipe Calderon told reporters at an event in Mexico City Trump's political rhetoric is "racist," and evocative of Hitler.
"This logic of praising the white supremacy is not just anti-immigration," Calderon said.
He added: "He is acting and speaking out against immigrants that have a different skin color than he does, it is frankly racist and [he is] exploiting feelings like Hitler did in his time."
Trump has accused Mexico of sending rapists and drug runners across the US border and has said he will make Mexicans pay for a wall on the border.
Former Mexican President Vicente Fox, Calderon's predecessor, also compared Trump to Hitler in an interview with Anderson Cooper on CNN.
"He reminds me of Hitler," Fox told Cooper.
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A growing rupture within the Republican party was on full display last night as Donald Trump boycotted a TV debate with his rivals in favour of holding his own rally for veterans.
Report by ITV News Washington Correspondent Robert Moore.
The mogul may have been absent, but his presence loomed large over the official Republican debate, said ITV News correspondent Robert Moore.
Meanwhile, Trump himself gave a typically brash and bombastic performance at his own rally to an enthusiastic reception.
When asked by ITV News whether he was glad to have skipped the chance to slug it out over policy with his competitors, he answered: "This has been a great night."
And would he watch the debate later? Trump hesitated before answering, "Yeah, I probably will."
Donald Trump has hailed his rally for veterans in Iowa as a "phenomenal night", saying more than $6 million (£4m) has been raised.
He said it was better than the debate where "they're all sleeping".
"Isn't that better than this debate that's going on where everyone's sleeping?" he said.