DETROIT (CBSNewYork/CBS News/AP) — Barbs began flying the minute the 11th Republican debate began Thursday night, with Donald Trump firing back at remarks leveled against him in recent days, and opponent Marco Rubio saying Trump had earned the personal attacks he has received.

But at the conclusion of the debate, all of Trump’s three opponents said they would make good on their promises to support him if he is the nominee.

Trump was joined for the debate at the Fox Theatre in Detroit by Florida Sen. Rubio, Texas Sen. Ted Cruz, and Ohio Gov. John Kasich. The debate was broadcast and moderated by Fox News Channel.

Each said at the end of the debate that they would support the Republican nominee even if was Trump, while Trump said he too would support another nominee if he does not win.

Rubio said, “I’ll support the nominee,” pointing out that the Democrats have one candidate, Bernie Sanders, who identifies as a “socialist,” and the other, Hillary Clinton, whom he claimed was under FBI investigation and “lied to the families of the victims of Benghazi.”

Cruz said he would also support Trump as the nominee, “because I gave my word that I would.”

And while Kasich said, “I kind of think that before is all said and done, I’ll be the nominee,” but he would also support Trump.

Trump incredulously said, “If it’s not me?” when asked if he would support someone else as the nominee, but he ultimately said he would do so.

PHOTOS: 11th GOP Presidential Debate

But the moment of relative unity at the end of the debate followed a string of acrimonious attacks – some of them sophomoric in tone.

On Tuesday, Trump racked up a huge number of delegates, winning several states, while Cruz won two contests in Texas and Oklahoma. Rubio came out the victor of the Minnesota caucuses.

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Co-moderator Chris Wallace began by asking Trump about remarks earlier in the day from former Republican nominee Mitt Romney.

Romney had warned Republicans Thursday morning to do whatever they can to nominate someone besides Trump, saying the current front-runner lacks the temperament and the integrity to be president.

“Here’s what I know. Donald Trump is a phony, a fraud,” Romney said. “He’s playing the American public for suckers: He gets a free ride to the White House and all we get is a lousy hat.”

In response, Trump said of Romney, “He was a failed candidate. He should have beaten President Obama easily. He failed miserably, and it was an embarrassment to everybody including the Republican Party.”

But far harsher attacks on Trump quickly came from his opponents on the stage. The moderators confronted Rubio about how he had mocked Trump’s tan and pointed out that he has small hands.

Rubio said Trump has “basically mocked everybody with personal attacks” all year, and “If there’s anybody who’s ever deserved to be treated that way, it’s Donald Trump.”

Within minutes of the debate, Trump even made a reference to the size of his penis in reaction to an earlier personal attack by Rubio about the size of his hands.

“He’s like 6’2”, which is why I don’t understand why his hands are the size of someone who is 5’2”. Have you seen his hands? They’re like this,” Rubio said earlier in the week. “And you know what they say about men with small hands? You can’t trust them. You can’t trust them.”

In response to the earlier comment, Trump ran with the sophomoric implications of the remark.

“Look at these hands? Are they small hands? He referred to my hands – if they’re small, something else must be small. And I guarantee you, there’s no problem,” he said.

Trump later slammed Rubio for calling him a “con artist.”

“This little guy has lied so much about my record,” Trump said.

Trump said the claim that his father gave him $200 million was false, and he had built a rich and powerful company out of $1 million. He said his career could serve as a model for the country.

Rubio further pointed out that the majority of primary voters have not picked Trump and alleged that Trump doesn’t represent the conservative movement, noting the businessman defended Planned Parenthood in the last debate.

But as CBS2’s Jessica Schneider reported, Trump defended his standing – pointing out that he has won 10 contests so far – amassing 324 delegates, while Cruz has 230 and Rubio 111. Trump said he is thus the best prepared to beat Hillary Clinton.

The exchange became so heated that moderator Megyn Kelly had to jump in, saying “no one can understand you when you’re talking over each other.”

Meanwhile, Cruz said the American people are not interested in listening to “bickering school children.”

He made the case in Thursday’s Republican presidential debate that voters want to hear the candidates talk about substantive issues, not insult one another.

Cruz made a pitch to the “truck drivers, steel workers and mechanics” who he says have been suffering under President Barack Obama the past seven years.

In a slam against Donald Trump, Cruz said it’s easy to print campaign slogans on baseball caps, but the question is whether he understands what made America great in the first place.

Cruz also accused Trump of using government power for private gain, and supporting liberal politicians for 40 years, “from Jimmy Carter to John Kerry to Hillary Clinton.”

Trump said he indeed had supported a variety of politicians of both parties and multiple ideologies, but he said the reason is that he is a businessman who knows how to get along with people.

“We need people to get along. We need to be able to get things done,” Trump said.

Trump further tried to present himself as a diplomatic businessman when Cruz pointed out that he had written 10 checks to Clinton – and four of them were for her presidential campaign in 2008.

Later in the debate, Rubio took issue with Trump and his since-shuttered Trump University, which is now the subject of a fraud lawsuit. In response, Trump dismissed Rubio as a poor politician who “wouldn’t get elected dog catcher.”

“The real con artist is Senator Marco Rubio, who was elected in Florida, and who has the worst voting record in the United States Senate. He doesn’t go to vote,” Trump said.

Cruz used the exchange to suggest that Trump would be a “disaster” as president.

“Let me just ask the voters at home – is this the debate you want playing out in the general election?” Cruz said.

He added, “If you don’t want him to be the nominee, then I ask you to stand with us as a member of a broad coalition.”

The candidates also sparred over an assortment of specific policy issues, including immigration. Trump said despite what he may have told the New York Times editorial board off the record, he is “not very flexible” when it comes to his proposals to build a wall across the southern border and force Mexico to pay for it.

Trump reportedly told the Times that he was flexible on some aspects of the plan. But he says many of those comments were off the record.

Still, he said “there’s always give and take, there’s always negation” when it comes to his policy plans.

Rubio noted that Trump could authorize the Times to release the interview transcripts if he wanted to.

But Trump said he was not doing it because he has “too much respect” for that process.

Trump later said the United States should engage in enhanced interrogation including waterboarding, and go further, even as senior military officials said they would not engage with him because of his proposed views on torture.

Waterboarding, the sensation of drowning, is illegal, but Trump said at Thursday’s GOP debate in Detroit that the U.S. should use waterboarding and worse in interrogations.

U.S. military and intelligence officials, some who’ve said they will not engage in such activity, are “not going to refuse me.”

“If I say do it, they are going to do it,” Trump said.

Cruz chided Trump for making bold statements, saying actions speak louder than words.

The moderators also took on Trump in particular and some of his policy and position statements. Moderator Kelly showed clips of Trump offering different positions on a host of issues, including Trump seemingly supporting allowing Syrian refugees into the country in a September interview then rejecting the same idea the next day.

Trump said he was generally unaware of the refugee issue the first time he was asked about it, but later found out the U.S. government planned to let “thousands” of refugees into the United States.

Also during the debate, Trump advocated flexibility on visas for highly skilled foreign workers.

For his part, Kasich said early on that he was the “adult on the stage” and said he will do better as primaries move to northern states like his native Ohio.

“I can get the crossover votes. Throughout this campaign I have talked about issues. I have never tried to get into scrums,” Kasich said. “People say wherever I go, you seem to be the adult on the stage.”

Pressed by a moderator about staying in the race to force a contested convention this summer, Kasich promised he will win Ohio on March 15 and will continue his message of “bringing people together.”

Kasich also said business owners should be allowed to refuse service to a same-sex couple getting married.

He said if a photographer refuses to take photos for a same-sex wedding, the couple should “find another photographer. Don’t sue them in court. The problem is in our country, we need to respect each other and be a little more tolerant for one another. Don’t go to court.”

Echoing a remark by Cruz on the issue, Trump also said he would rather have left the issue of same-sex marriage to the states.

Some questions also arose about local issues in Michigan, including the decline of Detroit over the past half-century and the water crisis in Flint.

Rubio slammed Democrats for “politicizing” Flint crisis, but he did not lay out any concrete ideas for preventing similar issues in the future.

He was asked why the GOP hasn’t seriously addressed the lead-poisoned water in Flint to the degree that it has been discussed by Democrats.

Rubio said Michigan Gov. Rick Snyder, a Republican, is holding people accountable for what happened in Flint. He said there is a “proper role” for government to play in such issues but did not give any specifics.

Outside the Fox Theatre, a sea of protesters lined the snowy streets chanting and holding signs slamming Trump.

Dan Leach of 97.1 The Ticket radio in Detroit said the protesters pushed up against the entrance gate, with signs reading “Dump Trump” and “Workers for $15.”

Fox News Channel announced its criteria for the debate last week. For candidates to qualify, they must have had at least 3 percent of support in the five most recent national polls published by Mar. 1 at 5 p.m. ET.

Detroit’s debate was also the first time Kelly came face-to-face with Trump in a debate setting after last year’s initial GOP forum, when the two got into a heated exchange over the front-runner’s treatment of women, CBS News reported.

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