* The first question of the night went to Donald Trump, who was asked to respond to Mitt Romney's criticism of him early in the day.
* Trump immediately came under heavy attack from Ted Cruz and Marco Rubio, who are both facing potential elimination if they don't start rallying more voters to their sides.
* Trump soon addressed comments about his "small hands." He made a not-so-thinly veiled reference to his genitals.
* Trump said that he will not ask The New York Times to release the audio from his editorial board meeting.
* Trump said that the military "won't refuse" his illegal orders to target terrorists' families and use torture.
* Rubio tried to make the case that he can rally the Republican establishment to his side. Cruz repeated that he has beaten Trump in more states than anyone else and has the conservative credentials to win.
* John Kasich struggled to break through during the debate.
The debate opened with Trump asked about Mitt Romney's remarks earlier today ...
The first question was about a speech Romney made earlier Thursday attacking Trump's candidacy and called him "phony."
Moderator Chris Wallace asked Trump, "He said your domestic policy will lead to recession, he's said your foreign policy will make us less safe, and then he listed what he said are your personal qualities. Quoting now Romney on Trump, quote, "the bullying, the greed, the showing off, the misogyny, the absurd, third-grade theatrics".
Trump repeated his earlier criticism of Romney: "Well look, he was a failed candidate, he should have beaten president Obama very easy.
"He failed miserably, and it was an embarrassment to everybody, including the Republican party. It looked like he went away on a vacation the last month. So, I don't take that, and I guess, obviously, he wants to be relevant. He wants to be back in the game."
... And soon there was a reference to the size of Trump's penis.
The moderators asked Rubio why he had chosen to use personal attacks against Trump in recent days.
Rubio responded, saying, "If anyone's ever deserved to be attacked that way, it's been Donald Trump, for the way he's treated people over the past campaign." He added that he hoped the debate tonight would be focused on policy, and blamed the media for giving oxygen to Trump's personal insults.
Trump, in turn, said he would take back calling Rubio a "lightweight," and then addressed Rubio's specific criticism of the size of his hands.
"He hit my hands," Trump said. "Nobody's ever hit my hands, I've never heard of this one. Look at those hands. Are they small hands? And he referred to my hands, if they're small, something else must be small, and I can guarantee there's no problem. I guarantee it."
Trump was also asked about off-the-record comments he made about his immigration policy.
Trump was asked at the debate about an audio recording, the existence of which was first reported by BuzzFeed News, of an off-the-record conversation Trump had with the New York Times which might show Trump wavering on his immigration views.
Trump responded by pointing out that BuzzFeed had claimed he would never run for president in the first place.
"I don't know what exactly what you talk about, when you talk about off-the-record. First of all, BuzzFeed, they were the ones that said under no circumstances will I run for president, and were they wrong. In fact, a lot of people said that," Trump said.
Trump continued, saying, "I did have a meeting with the editorial board of the New York Times, a very nice meeting. Many of those things were off-the-record, I think at their suggestion and my suggestion. And I think being off-the-record is a very important thing. I think it's a very, very powerful thing. I will say this, these three gentlemen have gone off-the-record many times with reporters, and I think they won't to honor it. And I will always honor it."
Trump said that there always has to be room for negotiation in order to get a good deal.
"Sometimes you ask for more than you want and you negotiate down to the point. I may have discussed something like that with the New York Times."
He added, "There's always give and take. There's always negotiation. And the best negotiator that knows what he's doing will make a great deal. But we need give and take in government. If you don't have give and take, you're never going to agree on anything. "
Trump was asked specifically about employing foreign workers at his hotels.
Asked about the H1B visa program, which is designed to allow U.S. employers to employ foreign professionals in speciality occupation, Donald Trump told Megyn Kelly he believed in increasing the number of visas.
"I'm changing," Trump said. "We need highly skilled workers in this country. And if we can't get it in this country, we'll bring them in."
Rubio shot back, critiquing Trump for hiring foreign workers for his hotels during busy seasons.
"You argue that you're here to fight on behalf of the America worker," Rubio said. "But when you have the chance to help the American worker, you're making your clothes overseas, and you're hiring your workers from overseas."
Cruz joined in: "You got U.S. companies that are firing American workers, bringing in foreign workers, and forcing them to train their replacements. And, I would note that is not dissimilar to what we discovered at the last debate concerning the hotel that Donald owns down in Florida. Down in Florida that hotel has brought in hundreds of foreign workers, and afterwards it was really striking."
Everyone noticed something … odd on Ted Cruz's lip.
Trump said that the military "won't refuse" his illegal orders to target terrorists' families and use torture.
Trump was pressed on some of his foreign policy proposals, including his plan to target terrorists' families and use torture, which foreign policy experts have said are illegal and would likely be refused by members of the military.
Trump's response: "They won't refuse. They're not going to refuse me. Believe me."
Moderator Bret Baier reiterated that his orders would be illegal. Trump responded, saying, "Can you imagine these people, 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."
Baier, following up, asked, "Even targeting terrorists' families?"
Trump, as he has done in the past, pointed to the families of September 11 attackers, saying they knew what was happening.
Rubio was asked about the Flint, Michigan, water crisis.
Rubio was asked why the GOP candidates have not done or talked much about the Flint, Michigan water contamination crisis.
Rubio said he and other Republican candidates have talked about the issue, but said the "politicizing" of the issue was unfair.
"What happened it Flint was a terrible thing," Rubio said. "It was a systemic breakdown at every level of government at both the state and partially the federal level as well."
He continued, "And by the way, the politicizing of it I think is unfair, because I don't think someone woke up one morning and said, 'Let's figure out how to poison the water system to hurt someone.' But accountability is important. I will say, I give the governor credit. He's taken responsibility for what happened."
Rubio added that Democrats have tried to turn it into a partisan issue against Republicans, calling their efforts "absurd and outrageous."
There was discussion about Trump University.
Trump defended the now-defunct Trump University amid multiple civil lawsuits claiming the program was a scam.
"Let's see what happens at the end of three years," Trump said, adding that he could have settled but insists he does not settle cases.
Rubio referenced the three pending lawsuits which include claims that the program advertised professors "handpicked" by Trump. According to the lawsuit, only one of the instructors had ever met Trump and that instructors had little or no experience in real estate investing.
"You're willing to say whatever you have to say to have them give you their money," Rubio said.
Trump maintained that he will win in court as 98% of the students were satisfied with the program, which shut down in 2010.
"He's trying to do to the American voter what he did to the people who signed up for the course," Rubio said.
Marco Rubio made a funny joke about yoga.
Cruz claimed Trump supported Kerry over Bush in 2004, but there is little evidence of that.
While Trump certainly had nice things to say about Kerry in 2004, he praised (and donated to) both candidates and publicly said he was undecided about the race. One biography of Trump says he supported Bush over Kerry.
And then, after all that, there was this:
Q: Will you support Trump as the nominee?
Trump was also asked if he would support his rivals. "Even if it's not me?" he incredulously joked. "Yes I will."
Also, there were hundreds of protesters outside the Fox Theater in Detroit during the debate.
BuzzFeed's World News editor Hayes Brown was tweeting and talking while watching the action.
Here's the current delegate situation:
Kyle Blaine, Jim Dalrymple II, Mary Ann Georgantopoulos, and Lisa Tozzi contributed reporting.