Donald Trump's rivals for the Republican presidential nomination all say they will back him if he wins the race.

"The only way you get anyone besides Trump, is through a brokered convention", Jowers said.

Romney did not mention that he accepted Trump's endorsement for his own presidential campaign in 2012, and praised the New Yorker's business acumen at that time.

In his speech at the University of Utah, Romney, who was the Republican presidential nominee in 2012, said that other GOP candidates - Florida Senator Marco Rubio, Texas Senator Ted Cruz and Ohio Governor John Kasich - would be suitable candidates for the presidency.

When asked about the timing of Thursday's speech, Romney said he simply "couldn't stand silent anymore" as the GOP race reached a "new low in presidential politics".

A win for Rubio on March 15 in his home state of Florida could raise questions about Trump’s strength, as could a win for Kasich, Ohio’s governor, on his home turf.

Mitt Romney said Friday he will not make an 11th-hour presidential run to try to stop Donald Trump from winning the Republican nomination, the day after sharply attacking the real estate mogul's candidacy. Romney's campaign self-destructed when he came off as being more interested in a 47 auto garage with an elevator than 47% of Americans.

Perhaps today's war of words between Mitt Romney and Donald Trump may be best explained in the immortal words of Taylor Swift: At this moment in time, it appears that the Republican establishment and the Trump wing of the party are never ever ever getting back together.

Nevertheless, it doesn't seem that Trump's support will weaken as a result of Romney's comments, KFOR notes. Some party leaders and donors are critical of Trump's positions on trade and immigration, including his calls to build a wall between the United States and Mexico, deport 11 million illegal immigrants and temporarily bar Muslims from entering the country.

Earlier, more than 70 Republican national security leaders signed a scathing open letter opposing Trump and his stance on many foreign policy issues.

"He doesn't have what it takes to be president", Trump said at a Portland, Maine, rally. "And his personal qualities would mean that America would cease to be a shining city on a hill". But I'm going to be supporting that person and doing everything I can to make sure they become the nominee of the party.

"I am the only one who can beat Hillary Clinton", Trump tweeted. "Hillary wants no part of "Trump"," he tweeted. "And Mitt Romney did not show this kind of courage because if he would have walked off the stage, like a Donald Trump would have done, he'd probably be president today".

Trump was dismissive of Romney's remarks. He has posted only anti-Trump messages on Twitter for the past week.