Nasty personal slights and playing loose with the truth but – in the end – pledges of personal fealty after the four remaining Republican hopefuls spent two hours mostly savaging each other in a Detroit debate Thursday night.

Donald Trump, 69, the bombastic New York billionaire who has built a formidable lead and a loyal following among citizens fed up with the dysfunction of Washington elites, was targeted jointly by Ted Cruz, 45, and Marco Rubio, 44. Those two U.S. freshman senators from Texas and Florida are battling each other to emerge as the "anti-Trump" alternative in a party deeply rent and in disarray.

Yet only hours after the 2012 Republican nominee, Mitt Romney, who lost to President Barrack Obama, called Mr. Trump a fraud unfit to be president, both Mr. Cruz and Mr. Rubio pledged they would loyally back the property magnate should Mr. Trump triumph as the party's presidential candidate.

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Mr. Romney denounced the front-runner "as a phony, a fraud," whose "promises are as worthless as a degree from Trump University" and who is "playing the American public for suckers." In return, Mr. Trump dismissed the former Massachusetts governor as a "choker" who begged for his support and managed to lose an easily winnable election to Mr. Obama.

That set the stage for a nasty slugging match – during which only Ohio Governor John Kasich, 63, who trails the winnowed filed of four, stayed above the fray – in which Mr. Trump was repeatedly stung but never floored by his younger rivals.

Mr. Trump repeatedly belittled his attackers as "Little Marco" and "Liar Ted." And in what may be a new low point in nationally-televised political debates, he mocked an earlier sexual slight from Mr. Rubio, who crudely suggested the some-time reality TV star had small hands – and by inference a small penis. Waving his hands aloft, the thrice-married Mr. Trump, declared: "I guarantee you, there's no problem" in that area," triggering yet another uproar in Detroit's historic Fox Theater.

He also fought off attacks over whether he was being hypocritical in his tough talk on trade wars and on rounding up and deporting millions of unlawfully resident foreigners because his companies have Trump-brand clothing made overseas and import temporary workers to staff his luxury resorts.

At times, Mr. Trump seemed rattled. At others he was entirely unfazed, even when it was suggested he was willing to resort to war crimes by torturing captured extremists and deliberately targeting their families with missile strikes. Asked what he would do if the U.S. military refused orders that would breach the laws of war, Mr. Trump said he would require obedience.

"They won't refuse. They're not going to refuse me," Mr. Trump said. Pressed as to whether that applied to even unlawful orders, he replied. "I'm a leader. I'm a leader. I've always been a leader. I've never had any problem leading people. If I say do it, they're going to do it. That's what leadership is all about."

He also offered a spirited defence of his vow to go beyond waterboarding – now-outlawed as torture. "These animals over in the Middle East that chop off heads, sitting around talking and seeing that we're having a hard problem with waterboarding, we should go for waterboarding and we should go tougher than waterboarding." Mr. Trump said.

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Mr. Trump also repeated his oft-discredited claim that some family members of the Sept. 11, 2001, hijackers were in the United States, had advance notice, flew home days before the attack and watched the aircraft slam into New York's twin towers on television.

"They left two days early with respect to the World Trade Center and they went back to where they went and they watched their husbands on television flying into the World Trade Center, flying into the Pentagon," he said.

Repeatedly the debate – the 11th since the Republican race for presidential nominee got under way last year with 17 hopefuls – degenerated into name-calling.

Mr. Rubio said Mr. Trump was a con artist, especially in connection with Trump University, which is currently embroiled in a class-action lawsuit by former students who claim they were bilked for worthless degrees. "He's trying to con people into giving them their vote just like he conned these people into giving him their money," Mr. Rubio said.

Angrily, Mr. Trump retorted: "Let me tell you the real con artist. Excuse me. Excuse me. The real con artist is Senator Marco Rubio … the people in Florida wouldn't elect him dogcatcher. He scammed the people of Florida. He scammed people. He doesn't vote. He doesn't show up for the U.S. Senate. He doesn't vote. He scammed the people. He defrauded the people of Florida."

Despite the debate-closing pledges that all four would back whichever one emerges as the party nominee, the next few weeks may prove even more bitter and divisive as both Mr. Rubio and Mr. Kasich face must-win primaries in their home states of Florida and Ohio. Both are big "winner-takes-all" states.