Sens. Marco Rubio and Ted Cruz, fighting for their political lives, relentlessly demeaned and denounced Donald Trump at Thursday's debate, all but pleading with Republicans to reconsider nominating a candidate with a long history of business failures, deep ties to the Democratic Party and a taste for personal insults.

Warning that Trump would lead the party to a historic defeat in November, Rubio and Cruz delivered their attacks with urgency, as if trying to awaken voters who had fallen under Trump's spell. Rubio derided Trump as untrustworthy and uncivil, while Cruz bashed him for donating money to Hillary Clinton's 2008 presidential campaign and to other Democrats.

But the debate in Detroit also deteriorated at times into the kind of junior high school taunts that have startled many Republican elders but done little to dent Trump's broad appeal.

At one point, as Trump and Rubio traded insults over their manhood, Trump recalled Rubio's innuendo that Trump's "small hands" correlated with another part of his anatomy.

Trump, who has boasted about his sexual exploits, insisted that nothing was small about him. "I guarantee you," he continued with little subtlety, "there's no problem. I guarantee you."

The two senators repeatedly urged Republicans to align against Trump in nominating contests over the next two weeks, saying that Trump could sew up the nomination even though a majority of voters so far have cast ballots for other candidates.

"Two-thirds of the people who cast a vote in a Republican primary or caucus have voted against you," Rubio told Trump. "The reason why is because we are not going to turn over the conservative movement or the party of Lincoln or Reagan, for example, to someone whose positions are not conservative."

The pleas reflected not only Trump's advantage in the race, but also the party's growing disquiet about the implications of nominating him. The specter of Trump as the Republican standard-bearer has long troubled both establishment-aligned and conservative leaders. But his initial hesitation to condemn the Ku Klux Klan in an interview Sunday, and his success in seven states on Super Tuesday, has set off a new wave of anxiety that Trump could tarnish the party this year and perhaps beyond.

Still, in a striking moment, all of Trump's rivals on stage indicated that they would support him if he became the Republican nominee. The consensus was especially unusual in the case of Rubio, who has been caustically attacking Trump as a "con man."

While Rubio savaged Trump repeatedly Thursday, Cruz combined his jabs with high-minded appeals to conservatives. He emphasized his support for a "simple flat tax" and a strong national defense, trying to position himself ahead of Rubio as the more competitive candidate against Trump.

Cruz also appealed directly to Trump's supporters by saying that their desire for a political outsider to lead the country was misplaced.

"For 40 years, Donald has been part of the corruption in Washington that you're angry about," Cruz said. "And you're not going to stop the corruption in Washington by supporting someone who has supported liberal Democrats for four decades, from Jimmy Carter to John Kerry to Hillary Clinton."

"Donald Trump in 2008 wrote four checks to elect Hillary Clinton president," Cruz added, turning to Trump to demand why he had done so.

"Actually, it was for business," Trump said, before noting that he had also given to Ronald Reagan and George W. Bush.

Trump found himself on the defensive throughout the night, challenged by his rivals and the Fox News moderators to explain his inconsistent stands in the past. He also had to defend himself against a movement begun earlier Thursday by Mitt Romney, the 2012 Republican nominee, who shredded Trump as a "phony" and a "fraud" who must be blocked from the nomination.

Trump, offered the chance to respond to Romney with harshness or with substance, chose the former.

"He was a failed candidate," Trump said. "He should have beaten President Obama very easy."

Cruz and Rubio, who have been grappling for ways to halt Trump's political momentum, seemed intent on trying to bait him into losing his cool. At one point, as Cruz tarred Trump for donating to Clinton's 2008 campaign, Trump tried repeatedly to interrupt.

"Count to 10, Donald — count to 10," Cruz said. Later, in an exchange over Supreme Court nominations, Cruz taunted, "Breathe, breathe, breathe — you can do it." That prompted Rubio to joke that his two rivals were primed for yoga, especially Trump. "He's very flexible," Rubio said, a quip referring to Trump's changes in political positions.

Rubio tried to get under Trump's skin by boring in on Trump University, the defunct education and training venture over which Trump is facing civil litigation, alleging that he defrauded students with false promises. Recalling that he had spoken to "one of the victims," Rubio said that what students had gotten in the courses was "stuff you could pull off of Zillow."

"Why won't you give them their money back?" Rubio asked.

Trump, who described the litigation as "a minor civil case," claimed that almost all students who had signed up for the courses "said it was terrific," but he quickly lost patience with Rubio. Calling him "little Marco" — a phrase he used several times — Trump noted that the senator was losing to him in Florida polls before the state's March 15 primary.

"The people in Florida wouldn't elect him dogcatcher," Trump said.

If Trump struggled to deflect the attacks on his character, business sense and political viability against Clinton in the fall, he seized opportunities to reassure conservatives that he would be a forceful commander in chief. Questioned by the moderators about his past advocacy for torture and for killing the families of terrorists, Trump stood firm and argued that "we should go tougher than waterboarding." Pressed about whether military officers would carry out such orders — killing terrorists' family members would violate the Geneva Conventions — Trump offered a boast.

"If I say 'do it,' they're going to do it," he said.

At another point, in a rare concession from Trump, he said he was "changing" his position on keeping more foreign-born college graduates in the United States so they could work here. He lamented that foreign citizens "go the best colleges" in America and "as soon as they are finished, they get shoved out" — the sort of language used by immigration reform experts with whom Trump often disagrees. His shift could appeal to some business leaders and moderate voters he would need in a general election.

"I am changing and softening the position because we have to have talented people in this country," Trump said.

Trump's shifting positions have been a target for his rivals for months, but during this debate, his rivals received help from the Fox News debate moderators. They played a compilation of video clips in which he was depicted changing his mind on issues like the war in Iraq. Trump was then asked directly if he had "a core."

"I have a very strong core, but I have never seen a successful person who wasn't flexible," Trump said.

One of the most anticipated faceoffs of the night was between Trump and moderator Megyn Kelly, who infuriated the candidate with her aggressive questions at the first Republican debate in August. Kelly and Trump breezily engaged each other Thursday night, but Kelly was pointed if polite in her questioning.

Both Rubio and Cruz repeatedly challenged Trump to release the full transcript of his meeting with The New York Times's editorial board earlier in the year. BuzzFeed reported this week that Trump, in off-the-record comments, had told the newspaper that he was willing to soften his hard-line immigration views.

But Trump stood his ground, saying he would "never release off-the-record conversations." Pressed by Cruz, he shot back: "I've given my answer, lying Ted. I've given my answer."

The fourth candidate, Gov. John Kasich of Ohio, who has positioned himself as the positive-sounding and seasoned executive in the race, largely stuck to that strategy. But Kasich, who is lagging far behind Trump and the other candidates, struggled to leave a mark in the debate other than portraying himself as the "adult" in the room.