The Republican presidential candidates, from left, Marco Rubio, Donald Trump, Ted Cruz and John Kasich stand for a moment of silence for former first lady Nancy Reagan prior to the debate

Texas Sen. Ted Cruz said Thursday night that Trump was guilty of 'funding liberal Democrats, and funding the Washington establishment,' and said the real estate tycoon couldn't be trusted to take on the federal government.

But he did it without raising his voice.

Republican National Committee chair Reince Priebus addressed the audience at the University of Miami before the debate began, using his time to calm the GOP's fraying nerves.

'This party is going to support the nominee, whoever that is, 100 per cent. There's no question about that,' he said.

Unspoken, but hanging heavy in the room, was the name of Donald Trump.

The billionaire front-runner has roiled the RNC with an unconventional slash-and-burn campaign style, trampling the party's more traditional candidates one by one since mid-2015.

The two 'establishment' candidates who remain – Florida Sen. Marco Rubio and Ohio Gov. John Kasich – are in third and fourth place among the quartet of candidates.

Republicans, Priebus insisted on Thursday night, 'are going to come together, unify in Cleveland, and get behind our nominee. That's what we do as Republicans.'

Cleveland will be the site of July's Republican national Convention.

'Any one of these four gentlemen would be a universe better than Hillary Clinton or a socialist like Bernie Sanders,' Priebus said.

Rubio, Florida's junior senator, received the night's largest ovation when he was introduced. But Trump set the tone for the night.