Trump had been aiming for a sweep of all five states, including North Carolina, Missouri and Illinois, that would have dealt another setback to establishment Republicans who fear his rowdy campaign will lead the party to defeat in November.

On the Democratic side, former secretary of state Hillary Clinton, 68, won in Florida, Ohio and North Carolina, as she put more distance between herself and rival Bernie Sanders, 74, a US senator from Vermont.

The wins for Clinton added to her lead of more than 200 pledged delegates over Sanders and will give her an almost insurmountable lead over the self-described democratic socialist.

“While we are on the right side this year, we will not be on the winning side,” Rubio, a first-term senator who is not seeking Senate re-election, told supporters in Miami as he pulled out of the race on Tuesday.

Rubio, 44, said the party’s establishment had long looked down on conservatives and taken their vote for granted.

“People are angry, people are frustrated,” he said, adding it would have been easy to stir up those frustrations and make people more angry. “I chose a different route and I’m proud of it.”

Trump’s closest challenger nationally is US Senator Ted Cruz of Texas, 45, a favourite of the conservative Tea Party.

The Trump loss in Ohio gives new hope to Republicans battling to deny the brash New Yorker the nomination and block him from capturing the 1237 delegates needed to win the nomination at the party’s July convention.

Trump, 69, said early on Tuesday that his momentum was already drawing in establishment Republicans who had previously baulked at his candidacy but now see him as the likely nominee.

“They’re already calling,” he told NBC’s Today show, without naming names. “The biggest people in the party are calling.”

AAP