Republican presidential candidates Marco Rubio, Donald Trump, John Kasich and Ted Cruz feuded over rhetoric, elections and immigration at the March 3 debate in Detroit. Here are the key moments. (Sarah Parnass/The Washington Post)

Billionaire Donald Trump entered Thursday night’s GOP debate as the race’s front-runner — but he spent much of the night on the defensive, struggling to explain his positions to skeptical moderators, arguing with his rivals, even trying to drown out their arguments with shouted insults.

“I won 10 states,” Trump said at one point, reasserting his dominance on a night when it seemed to be under assault. “I am by far the leader!”

Throughout the debate, both Sen. Marco Rubio (Fla.) and Sen. Ted Cruz (Tex.) returned to the furious attacks they had mounted on Trump a week before. Rubio, as before, assailed Trump with an eye toward moderate voters — asserting, again and again, that he was an unserious con man who was simply telling them what they wanted to hear. Cruz made a different pitch: Aiming at conservatives, he repeatedly sought to assert that Trump was a closet liberal, who had donated and befriended conservative enemies like Democratic front-runner Hillary Clinton.

Trump replied, as before, that he was beating them both. Which he is. With the anti-Trump vote still split between Cruz, Rubio and Ohio Gov. John Kasich, it will be hard for a single challenger to pass Trump.

“Millions and millions of people have come to the Republican Party in the last little while,” Trump said at the debate’s end.

Thursday’s debate came at a crucial point in this entirely unexpected GOP primary season.

Trump dominated the primaries of Super Tuesday this week, and now he has a significant lead over his top rival, Cruz, in the race for Republican convention delegates.

But in a divided field, Trump has still won less than half of all the delegates awarded so far. That leaves his opponents with a viable — but risky and destructive — strategy. The only way to stop Trump from winning the nomination may be to stop anyone from winning it: dividing up the delegates so that no one has a majority, forcing a brokered convention.

The next states to vote will be Kansas, Kentucky, Louisiana and Maine, all on Saturday. After that, the next primaries will be Tuesday, when Republicans vote in Hawaii, Mississippi, Idaho and Michigan, which is why Thursday’s debate was in Detroit.

The debate reflected the degree to which Trump has changed the GOP’s discourse – at one point, he made an unprompted joke about his own genitals — but also the degree to which the other candidates have mimicked his style. Cruz often treated Trump like a child with a temper tantrum, urging him to “breathe” with mock concern. Rubio repeatedly interrupted Trump, as Trump had interrupted others, saying, “False. False,” as Trump tried to make a point.

Here’s an at-a-glance look at the candidates’ views on the day’s biggest issues.

Kasich, as he did in the last debate, did not participate in the attacks on Trump. Instead, he seemed to be holding his own private event at the side of the stage, ignoring the fighting next to him and trying to speak directly to voters.

At the end of the debate, all four candidates onstage refused to break the last taboo of a party debate. The other three said they would vote for Trump, if he became the GOP nominee. Trump said he would vote for one of them, if the nominee turned out to be somebody else — a vow he has made, and then reconsidered before.

But first, Trump mocked the idea that he might have to face the choice at all.

“Even if it’s not me?” he asked, as if the idea were something he hadn’t thought of before.

For most of the night, Trump was on the defensive. At one point, he resorted to shouting insults, seemingly trying to drown out an attack by Rubio. “You’ve defrauded the people of Florida, little Marco!” Trump repeated, after Rubio (Fla.) went on a riff about Trump’s involvement in the troubled “Trump University.” That was not a school, but rather a series of real-estate seminars, which later lawsuits alleged was a scheme to bilk students out their money.

Cruz later interrupted to turn their argument itself into evidence against Trump.

“Let me just [ask] the voters at home: Is this the debate you want playing out in the general election?” Cruz said. “If we nominate Donald, we’re going to spend the fall and the summer with the Republican nominee facing a fraud trial,” Cruz said, referring to an ongoing suit which may come to trial this summer.

Trump interrupted again: “Oh, stop it. . . . It’s a minor civil case.”

Cruz spoke to him as he would speak to a child. “Donald, learn not to interrupt. It’s not complicated. Count to 10. Count to 10,” Cruz said.

Hours before the debate, Trump was attacked not by a rival, but by a former ally. Mitt Romney — the 2012 Republican presidential nominee — called him a “phony” and challenged Trump to campaign without insults.

“Well, look, he was a failed candidate,” Trump responded during the debate, speaking of Romney, who he endorsed in 2012. “He failed miserably.”

That, it turned out, was the most high-minded portion of the early minutes of the debate. Within its first 10 minutes, Trump had made what may have been the first reference — although a slightly veiled one — by a presidential debater to his or her own genitals.

“He hit my hands. Nobody has ever hit my hands,” Trump said, referring to Rubio, who had indeed said that Trump’s hands were small in recent days, and intimated another part of Trump’s anatomy. Trump noted that: Rubio, he said, had implied “Something else must be small.”

Trump spoke to the national TV audience. “I guarantee you there’s no problem,” Trump said.

Trump — having defied nearly every other unwritten rule of presidential campaigns — spent part of Thursday evening defying perhaps the greatest of all: He gave an impassioned defense of flip-flopping.

“I’ve never seen a very successful person who wasn’t flexible. Who didn’t have a certain degree of flexibility. . . . You have to be flexible. Because you learn,” Trump said, after a video montage in which Fox moderators showed him changing his mind about the Iraq War, about whether to admit Syrian refugees, about whether President George W. Bush had lied to the American public. Trump, in essence, faced the kind of accusation that had burdened past nominees like Mitt Romney. And he said: why not? “You have to show a degree of flexibility. If you’re going to be one way, and you think it’s wrong, does that mean the rest of your life” you can’t change?

The crowd cheered.

Rubio sought to turn that into a mistake.

“There’s a difference between flexibility and telling people whatever you think you need to say to get them to do what you want them to do,” Rubio said, repeating an attack line he used several times during the night. He added: “You are willing to say whatever you had to say in order to get them to give you their money. . . . And we’re not going to do that with our country.”

During the debate, Trump reversed himself on a key part of his own immigration platform — calling for an increase in visas for highly skilled foreigners during Thursday night’s debate, even though his own campaign website still calls for the opposite.

“I’m changing. I’m changing. We need highly skilled people in this country,” Trump told Fox News moderator Megyn Kelly, after she pointed out that Trump’s official campaign policy was still — according to the website — that there should be fewer visas given to highly skilled foreigners. “We absolutely have to keep the brainpower in this country. I’m changing it, and I’m softening the position.”

Trump also, at a later point in the debate, defied another GOP orthodoxy: that Russian President Vladimir Putin was an impacable, wily, untrustworthy U.S. enemy. Instead, Trump — whom Putin had praised in the past — said that he thought he could get along with the Russian leader.

“Putin said very nice things about [me]. And I say, very nicely, ‘Wouldn’t it be nice if we could get along with Russia?’ ” Trump said.

After months of flailing and attacking one another, Cruz and Rubio seemed to advance a relatively coherent argument against Trump — arguing that, in private, he had hurt the very blue-collar workers he now wants to represent.

Rubio pressed him about the fact that Trump-branded clothing is often made overseas, and about the widespread hiring of foreign workers on visas at one of Trump’s properties in Palm Beach, Fla.

“You’re making your clothes overseas, and you’re hiring your workers overseas,” Rubio said.

To defend himself, Trump sought to explain the economics of owning a Palm Beach resort. “It has a very short season. It’s called ‘The Season,’ ” Trump said, saying that Americans were not interested in short-term work. “Other hotels do the exact same thing.”

Cruz, for his part, pressed Trump about reports that he had given an off-the-record interview to the New York Times editorial board in which Trump supposedly signaled flexibility on his hard-line positions on immigration.

Cruz pressed Trump to tell the Times to release a record of the off-the-record interview.

Trump refused. Earlier in the evening, he had said his respect for the press was too great. “I have too much respect for that process to say, ‘Just release that,’ ” Trump said.

“If, in fact you went to Manhattan and said, ‘I’m lying to the American people . . . ’ ” Cruz said.

“I’ve given my answer, Lyin’ Ted,” Trump said.

At times, prodded by the moderators, the debate turned for short periods to questions of policy. At one point, a moderator asked Trump about an interview with former National Security Agency and CIA director Michael Hayden, in which Hayden said the U.S. military might refuse orders that Trump has contemplated, including plans to kill the family members of Islamic terrorists.

“They won’t refuse. They’re not going to refuse me,” Trump said. He added, “If I say do it, they’re going to do it.”

Moderator Chris Wallace had one of most powerful moments of the early going, pressing Trump to explain a claim that he would save $300 billion from Medicare drug purchases, when the U.S. only spends $78 billion total on Medicare drug purchases. Trump seemed to dodge the question, despite Wallace’s repeated efforts to pin him down.