At the CNN debate in Miami, GOP candidates sparred over immigration, social security, how to talk about Muslims and more. Here are the key moments. (Sarah Parnass/The Washington Post)

They had tried criticizing him, interrupting him, insulting his tan and mocking his fingers. On Thursday night — after nothing else worked — the rivals of Donald Trump seemed to try another tactic: They would try . . . nothing at all.

The result was a low-key, civil debate, where the Republican front-runner was often given free reign to muse. In some cases, that meant contradicting himself — Trump excoriated visas for high-skilled workers but said he used them. He lamented that Democrats would not change Social Security and then said he, too, wouldn’t change it. He attacked the current system of big-money campaign finance but then said that as a donor he had used it — and might soon use it again, by possibly soliciting big donations as a general-election candidate. He said he wanted to be neutral in negotiations between Israelis and Palestinians. Except that he would be really pro-Israeli. Except that he’d say he was neutral so that the Palestinians would trust him.

At one point, Trump hinted that he would change his mind far more often as president, hinting that he would be “flexible” on a variety of topics after a campaign built on hard-line, taboo-breaking proposals. A moderator asked: Flexible about what? “It depends on what comes up,” Trump said.

His rivals rarely pointed out when Trump had contradicted himself. The moderators often let him off without follow-ups, in contrast to the probing questions Trump got from Fox News last week.

Even Trump himself seemed surprised. “So far, I cannot believe how civil it’s been up here,” he said at one point.

Would civility be the thing that finally took Trump down? It doesn’t seem likely. But nothing else about this campaign has, either.

“Sometimes, being positive isn’t all that interesting,” said Ohio Gov. John Kasich at the debate’s end. Kasich, like the others, had largely spared Trump as the night went on. Now, he was making his final closing argument before his home state votes Tuesday, and he was admitting that many people might not find him very exciting. “But it’s very interesting to my family” and friends, Kasich said.

The other two candidates onstage — Sens. Ted Cruz (Tex.) and Marco Rubio (Fla.) — seemed to take on Trump only on the issues, not his character.

When Trump said that he does not condone violence against protesters at his rallies — but also seemed to excuse the anger that motivated them, Rubio said: “I’m concerned about violence in general in this society.”

Kasich’s criticism was relatively oblique: “Elections are important. But the unity of this country really matters.”

And Cruz turned his attack instead on President Obama. “We’ve seen for seven years, a president who believes he’s above the law, who behaves like an emperor,” Cruz said. He turned to the crowd: “How many of y’all feel disrespected by Washington. Washington isn’t listening to the people. And that’s the frustration that is boiling over.”

That seemed to reinforce Trump’s statement when he was asked about an altercation at a rally between a protester and one of his supporters, who on Thursday was charged with assault.

“People come with tremendous passion and love for the country. And when they see protesters,” Trump said, then trailed off. “When they see what’s going on with this country, they have anger that is unbelievable.”

He was asked by a moderator about statements the candidate himself had made from the stage, suggesting that protesters might be punched, or roughed up by the crowd. Trump responded by saying that the violence was often started by protesters.

“We have some protesters who are bad dudes. They have done bad things. They are swinging, they are really dangerous, and they get in there and they start hitting people,” Trump said.

Earlier, Trump gave an unusually wide-ranging denunciation of Islam and Muslims in Thursday’s GOP debate, saying that “a lot of ’em” hate America.

“I will tell you, there’s something going on that maybe you don’t know about, maybe a lot of other people don’t know about, but there’s tremendous hatred,” Trump said, after he was asked about a comment he made this week that “Islam hates us.”

Rubio criticized Trump in his most forceful attack of the night, saying that Trump’s comments would hurt America’s interests by alienating Muslims overseas. Trump stood by it, and expanded with his own criticism of Islam’s treatment of women.

“You can be politically correct if you want. I don’t want to be so politically correct. I like to solve problems,” Trump said. “Islam. Large portions want to use very, very harsh means. Let me go a step further. Women are treated horribly. You know that, you do know that.”

That brought a rebuttal from Cruz, who had mocked Trump several times in the debate with caveman-like over-simplifications of policy arguments.

Islamic terrorism was a huge threat, Cruz said, and he blamed President Obama for under-playing and under-estimating the threat. “That is maddening,” Cruz said. “But the answer is not simply to yell, ‘China bad! Muslims bad!’”

Then Cruz turned to an argument that, in essence, Trump’s harsh rhetoric belied his actual policy positions on the Middle East, which Cruz believed were not hard-line enough. For instance, Cruz believed that Trump was not sufficiently pro-Israel and would give away too much by seeking to be a neutral broker in future talks between Israel and Palestinians.

Prior to the exchange, the most spectacular arguments were between Trump and — himself. In two instances, Trump outlined a policy that he said was bad — and then explained how he himself embraced it.

On the question of immigration, for instance, Trump said that the system of H-1B visas, meant for highly skilled foreigners, was harmful to U.S. workers. But he still uses it at his businesses.

“It’s something that I frankly use. And I shouldn’t be allowed to use. And we shouldn’t have it,” Trump said of the H-1B program. “It’s sitting there waiting for you. But it’s very bad . . . for our workers. And it’s very unfair for our workers.”

And then, on the subject of Social Security, Trump seemed to criticize Democratic candidates Hillary Clinton and Bernie Sanders for saying they didn’t want to change Social Security, and even wanted to expand it.

Trump then said that he, too, did not want to change Social Security.

“I will do everything in my power not to touch Social Security, to leave it the way it is,” Trump said, after Rubio had said he wanted to gradually change Social Security so that future generations of retirees would retire later. “It’s my absolute intention to leave Social Security the way it is.”

It was an almost post-apocalyptic debate, in which Trump rivals who had been blasting him a week earlier seemed to see little point in fighting anymore. As the debate went on, Trump even seemed to turn the heat up gradually against Cruz, saying that he had supported “amnesty” for undocumented immigrants. It was one of the worst insults of this campaign so far. Cruz just laughed, and the debate moved on. At another moment, a moderator noted that Cruz had mocked Clinton for saying something that Trump had just said right in front of him — that Social Security could be salvaged by cutting only ‘Waste, fraud and abuse” — and not any benefits that people actually need.

Did he just compare Trump to Hillary Clinton? A moderator asked.

Cruz, even then, didn’t bite. “I will let Donald speak for himself,” Cruz said.

Social Security is a major issue in Florida, where Thursday’s debate was held. That the debate was held in Florida was recognition that the biggest prize at stake on Tuesday is the Sunshine State, where 99 delegates will be awarded to the winner, regardless of the voting percentages.

On Thursday, a Washington Post-Univision News poll showed Trump leading Rubio by 38 percent to 31 percent among likely Republican voters in Florida. That actually is good news for Rubio: Previous polls have shown him losing to Trump by double digits. But still, he would come away empty handed if he lost his home state.

On another key issue in Florida, climate change, Rubio took a strong stance, although one that may not prove popular in Miami. He rejected the idea of adopting new policies to fight climate change, saying that the U.S. government could do nothing that would make a difference.

Rubio was asked by a moderator about a statement from Miami’s mayor, a Republican who has endorsed Rubio, worrying that rising sea levels would swamp parts of his city. Rubio responded with skepticism that climate change was really a man-made phenomenon, saying that the climate was always changing. Rubio then pivoted to a slightly different argument — saying, in essence, that U.S. policies could not stop climate change because other countries were already pumping out so many greenhouse gases.

“I am not going to destroy the U.S. economy for a law that will do nothing for the environment,” Rubio said. It was a remarkable contrast from the Democratic debate the night before, when both Democratic candidates spoke about climate change as an urgent problem and articulated an urgent need to reduce greenhouse-gas emissions.

Kasich responded by saying, “I do believe” that man-made climate change is occurring. Trump and Cruz were not asked their opinion.

Thursday night’s debate contrasted sharply with previous few weeks encounters, when the GOP nominating process became surreal as Trump’s challengers turned desperate.

Rubio, in particular, had sought to copy Trump’s insult-comic style, mocking the front-runner’s tan and insulting the size of his fingers. Trump responded in the next debate with an even more surreal moment: to reassure anyone who would draw implications from the size of his hands, he volunteered onstage that there was “not a problem” with the size of his genitals.

If that moment hurt Trump, it didn’t hurt him much: He has won five of the seven states that have voted since then.

Losing Florida would be devastating not just to Rubio’s presidential campaign, but also to his political career. Rubio is leaving the Senate, and he would have to face the next phase of his life with Trump’s epithet “Little Marco” metaphorically hung around his neck.

For Kasich, who as in previous debates tried to stay out of the fighting, the firewall is his home state, Ohio, which will be almost as big a prize as Florida, with 66 delegates for the winner and zero for everybody else. Polls show Kasich in a close race with Trump in Ohio, where the GOP apparatus is strongly behind the governor and he has a fairly high approval rating. If he can beat Trump anywhere, Ohio is it.

Trump so far has won GOP contests in 15 states. He has accumulated about 458 Republican delegates, 99 more than Cruz, his closest rival. Trump needs 1,237 delegates to win the nomination.

During the debate, Trump said he would be the best at undoing the damage done to American workers by foreign trade deals — and his argument was that, as a businessman, he has exploited those very laws skilfully.

“Nobody knows the system better than me,” Trump said when debate moderator Anderson Cooper asked about Trump’s hiring of foreign workers for his businesses. Trump also has been criticized for having his name-branded clothing and other products made overseas, despite his campaign-trail rhetoric bemoaning the loss of American manufacturing jobs. “I will take advantage of it — they’re the laws. But I’m the one who knows how to change it,” Trump said.

The subject of foreign trade has become a powerful issue in both the Republican and Democratic presidential primaries, with Trump and Democratic-primary candidate Bernie Sanders appealing to white working-class voters by blaming long-serving politicians for the trade deals that shipped jobs away.

In Thursday’s GOP debate, the two candidates who are in the Senate sounded strongly skeptical of deals that allow freer trade, which have long been favored by Republicans in Washington. Both accused the Obama administration of essentially failing to hammer out trade deals that worked well and failing to properly enforce what had been agreed to before.

“There are great trade deals, and there are bad ones,” Rubio said.

“We’re getting killed in international trade right now,” Cruz said.

Another Washington deal issue came up during the debate: Cuba.

Trump appeared to make up his policy on Cuba on the spot, pausing for a moment to decide that he would undo President Obama’s decision to reopen the U.S. Embassy in Havana.

“I would probably have the embassy closed,” Trump said, after being pressed by CNN’s Dana Bash. He said the closure would be temporary, while he renegotiated the terms of America’s rapprochement with Cuba. “Make a deal. It would be great. But it’s got to be a great deal. Not a bad deal by the United States.”

What followed was one of Rubio’s best moments of the night — a chance to defend a hard-line policy toward Cuba’s communist regime, which was a key part of his platform during his rise in Florida politics. Rubio is the son of Cuban immigrants.

“Here’s a good deal. Cuba has free elections. Cuba stops putting people in jail” for political reasons, Rubio said, listing the details he would demand in a future deal with the island’s communist government. “You know what? Then we can have a relationship with Cuba. That’s a good deal.”

Trump opened the debate by saying that his party’s establishment should embrace him — not fight him — because he is bringing new voters into the primary process.

“They’re voting out of enthusiasm. They’re voting out of love. Some of these people, frankly, have never voted before,” Trump said, calling his success in the GOP primaries one of the biggest political stories around the globe. He said that the GOP establishment should accept him, because he could defeat a Democrat in the fall: “We’re going to beat them soundly.”

Earlier in the day, reports indicated that Trump will soon be endorsed by a candidate who was involved in earlier main-stage debates — retired neurosurgeon Ben Carson.

Later in the debate, Trump was asked whether he would give up a pledge to “self-fund” his campaign if he became the Republican nominee and needed an influx of cash to take on a Democratic candidate.

“I have not made that decision yet,” Trump said. “My decision was that I would go through the entire primary season” without taking large contributions, Trump said. Despite his rhetoric, he is still taking donations, including from people who use the “Donate” button on his website.

He later condemned the very system he was now considering joining. “They make large contributions to politicians, and they have total control over those politicians. I don’t want anybody to control me, but the people right out there” he said, meaning the debate audience.

Trump then returned to a tactic he employed several times Thursday night: citing his political impurities as a private citizens as proof that he can purify the system as a politician.

“I know the system is broken,” Trump said, speaking of campaign finance as someone who has given large amounts to politicians from both parties. “Frankly, I know the system better than anybody else. And I’m the only up here who’s going to be able to fix that system.”

Toward the end of the debate, Trump seemed to sum up his approach to governing in one sentence: despite his promises of a hard-line stance on a variety of subjects, he would be flexible when actually setting policy. Flexible on what? “It depends on what comes up,” he said.