The night started with a joke about the size of Trump's genitalia - and went downhill from there, often devolving into hard-to-comprehend shouting matches, with Trump repeatedly interrupting his rivals and firing off endless attacks of his own.
At one point, Cruz coached Trump to "breathe, breathe, breathe".
That prompted Rubio to make a joke about yoga and call Trump "very flexible", a nod to the frontrunner acknowledging that he has changed his position on assault rifle bans and visas for foreign workers.
This is what the Republican race and, especially, the debates have become - and there's no sign that the slugfest will end, even as Trump wins more and more primaries.
Trump took the stage with a tight smile and a wave that turned into a thumbs-up. His wife had warned him not to get too nasty in his attacks, to act "presidential", but Trump said he responded: "When you have incoming, you can't be too presidential."
The opening question of the debate forced Trump to again respond to Romney's accusations that the country would suffer with him as president - namely his domestic policy would lead to a recession and his foreign policy would make the country less safe. Romney also listed Trump's personal qualities, which the moderator rattled off as Trump frowned. It was a concern that Rubio and Cruz would echo again and again.
"The bullying," Fox's Chris Wallace said, as the crowd cheered. "The greed. The showing off. The misogyny. The absurd, third-grade theatrics."
"Well, look, he was a failed candidate," Trump said of Romney. "He should have beaten President Obama very easy. He failed miserably, and it was an embarrassment to everybody, including the Republican Party."
Trump quickly pivoted to talking about trade and how he would improve it as president, but Wallace brought him again back to Romney's attacks, asking Trump to share his views on the Ku Klux Klan and white supremacists.
Trump quickly disavowed the KKK and its former leader, but he slammed the media for continuing to bring this question up.
Throughout the debate, Trump was clearly unhappy with the way he was being treated by his fellow Republicans, though, at the end, they swore that they would support him if he becomes the nominee.
On a day when most candidates lock themselves away with top aides for a final round of debate prep, Trump started his by calling into the morning shows to pre-empt an expected attack from Romney - and then he flew to Maine to hold a rally where he could counterattack Romney while getting some face time with voters ahead of the state's contest tomorrow.
The Romney attack hit Trump differently from those before it, and he responded as if he had been deeply personally hurt and offended. Trump has repeatedly said that he got into the race because Romney let him down by losing, and he decided he would rather win an election and run the White House himself than trust anyone else to do it.
At the Maine rally, Trump said that Romney begged him for an endorsement in 2012 and "would have dropped to his knees" if Trump had asked him to do so. Trump said he then recorded robo-calls for Romney and hosted a fundraiser for the "stiff" that attracted so much attention that he had to host two sessions on a rainy day.
"Everybody's shoes were so wet, I ruined my carpet," Trump said in disbelief at the rally.
"This carpet was wiped out, and nobody thanked me for the carpet. Hey: Maybe I can send Mitt a bill for carpet ruined."
At the debate, nearly every question, including those that went to other candidates, put Trump on the defensive.
He mocked his rivals, talked over them as they answered questions and even assured the crowd that he doesn't have small hands - as Rubio has said - and he can "guarantee" other body parts are not small, either.
The Fox News moderators came prepared with questions that seemed to be written with Trump's expected answers in mind, along with full-screen graphics showing that Trump's proposed budget cuts would not yield nearly as much money as he has promised.
Cruz and Rubio, who in previous debates had gone after each other, both locked on Trump instead. They pushed him for specifics on his policy proposals and reprimanded him for his personal attacks.
Rubio suggested that someone who doesn't answer questions can't be president, while Cruz said that the "stakes in this election are too high" for Trump to be the nominee.
Trump had to defend several of his business deals - Trump University, which is caught up in lawsuits alleging fraud; his clothing line, some of which is made in other countries; his Florida resorts, which hire foreign workers during the busiest season instead of hiring locals.
But Trump seemed to become especially agitated when Cruz and Rubio implied that the billionaire couldn't win the nomination or the election. It was an attack that just didn't seem to make sense to Trump.
"Just for the record, I have won 10 [states]. He has won three or four," Trump said, referring to Cruz.
"Last week, in fact, on Tuesday, I was a half a million votes higher than him.
"I was a million votes higher than Marco, one million votes. That's a lot of votes."
The 11th Republican debate also marked a return engagement for Trump in his off-and-on feud with Megyn Kelly, one of the co-moderators. She asked him about the possibility that he might change his immigration policies once he gets into office, citing an off-the-record meeting with the New York Times that was reported by BuzzFeed.
Rubio and Cruz tag-teamed Trump, insisting that he give the newspaper permission to release the interview.
"He has a very simple solution," Cruz said.
"Simply release the tape."
-Bloomberg
