And, in another history-making exercise, Romney all but endorsed something of a contested national convention this summer.

Last night the four candidates including Marco Rubio, Donald Trump, Ted Cruz and John Kasich were involved in a tough debate that at times became very dirty indeed.

Along with describing Trump as a "phony" and a "fraud", the 2012 GOP presidential candidate will tell viewers that the NY businessman is "playing the American public for suckers".

And then there was an exchange about Trump University - the defunct program Trump ran to teach attendees the secrets to success.

But it's hard to imagine leading Republicans, such as Mitt Romney and Nebraska Sen. Others welcomed him to the Trump-bashing club. "The answer is, yes I will", he said.

Like Romney, she finds Trump the worst thing in the world until he wins the nomination, at which point more prosaic partisan considerations will again prevail. The cuts of his record on amnesty, the Affordable Care Act, abortion, bailouts, and global warming might make you think that Romney is running in the 2016 campaign. In an interview with Yahoo News' Katie Couric, Carson said Romney, as the former standard-bearer for the party, should not have weighed in as negatively as he did. "If we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished", he said, warning that the kind of anger Trump has displayed has led other countries "into the abyss".

With Trump emerging as the undisputed GOP front-runner, many party leaders and insiders insist he's not a true conservative.

He said that Mr. Trump has told voters that he will betray them when he repeatedly said during the debate Thursday that as a deal-maker you "have to be flexible". Trump countered with a feint, saying he'd called Rubio a "lightweight" in the past but "he's really not that much of a lightweight".

With the huge number of tuning in to hear and watch the Republican debate, there is no surprise that candidates do their best to impress the audience to win more supporters.

A raucous Republican Party debate on Thursday drew the biggest audience for a televised US presidential match-up in 2016, according to early ratings data, but viewership was well below the record set last August. Rubio said he would rather talk about issues, and Trump agreed.

"In the past, our presidents have channeled that anger, and forged it into resolve, into endurance and high objective, and into the will to defeat the enemies of freedom", he said.