Senator Marco Rubio, who after the departure of Jeb Bush from the campaign was regarded as the Republican establishment's best hope for beating maverick candidates Ted Cruz and Donald Trump, has announced his capitulation saying it was "not God's plan" for him to be President in 2016.

Rubio had failed to impress conservative voters because of his perceived weakness on immigration and a series of campaign gaffes. He had not showed himself to be strong in debate, a failing ruthlessly exposed by another former candidate, Chris Christie, and he was forced to apologise for an attempt to 'out-Trump Trump' with a vulgar physical reference. Rubio also adopted a high-risk strategy in early-voting states like Iowa which saw him spend less time on the ground than other candidates.

He said after yesterday's primaries: "While it is not God's plan that I be president in 2016 or maybe ever, and while today my campaign is suspended, the fact that I've even come this far is evidence of how special America truly is and all the more reason why we must do all we can to make sure this nation remains a special place."

However, his departure has left the Republican party even more fractured. Front-runner Donald Trump swept three states, but the New York billionaire's loss in the crucial state of Ohio wrought more chaos for a party deeply damaged by his candidacy.

While the Republican race remained in turmoil on Tuesday, Hillary Clinton's victories in Florida, Illinois, Ohio and North Carolina cast doubt on US Senator Bernie Sanders' ability to overtake her for the Democratic Party's nomination.

Trump's wins in Florida, Illinois and North Carolina brought him closer to the 1,237 delegates he needs to win the nomination and left those in the party trying to stop him with a dilemma.

Republicans can either throw their weight behind a candidate who rejects their policy goals or go on trying to stop him in the hope that he falls short of the majority required, enabling them to put forward another candidate at the July convention in Cleveland to formally pick their candidate for the November 8 election.

That, however, would risk alienating the millions of Americans who back the real estate developer and former reality TV show host.

Ohio Governor John Kasich's victory in his home state left him as the last establishment Republican candidate standing after Rubio pulled out after losing in a Trump landside in his home state of Florida.

US Senator Ted Cruz of Texas, who has sought to fashion himself the lead anti-Trump alternative, was outclassed by Trump everywhere on Tuesday except Missouri, where he trailed Trump narrowly with nearly all votes counted.

Many party leaders are appalled at the billionaire Trump's incendiary rhetoric and believe his policy positions are out of step with core Republican sentiment, such as his vow to deport 11 million illegal immigrants, temporarily ban Muslims from the United States, build a wall along the border with Mexico and impose protectionist trade policies.

Additional reporting by Reuters.