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)

There was an unfamiliar buzz on the debate stage here Thursday night: the sound of Republican presidential candidates engaging in a sober discussion of policy, rather than savaging each other.

Their 12th debate took a markedly different tone as Donald Trump’s remaining three rivals prepare for a crucial round of primaries next week that could represent their last chance of stopping him on his march to the GOP nomination.

While there were sharp exchanges, they were over Social Security, visa programs for foreign workers, how to fix the veterans’ health-care system, policy toward Cuba and the merits of free trade deals. No one mentioned “Little Marco,” “Lyin’ Ted” or the size of anyone’s hands.

“We’re all in this together,” Trump said. “We’re going to come up with solutions. We’re going to find the answers to things.

“And, so far, I cannot believe how civil it’s been up here,” the celebrity billionaire marveled.

That was because each of them has something to prove and little time to do it.

Trump sought to project a command of issues and a temperament that is suited to the Oval Office, rather than a reality show.

Sen. Marco Rubio of Florida was attempting to repair the damage that he has done to his reputation, and his presidential prospects, by baiting Trump with schoolyard taunts.

Sen. Ted Cruz of Texas portrayed himself as an outsider, like Trump — but one with greater intellectual depth.

And Gov. John Kasich of Ohio emphasized his blue-collar roots and his governing experience — the latter commodity being one that thus far has not found a market in this year’s discontented electorate.

Trump’s opponents drew pointed yet substantive contrasts with the front-runner over his view that many Muslims around the world “hate” the United States. Cruz, Rubio and Kasich argued that Trump’s rhetoric unnecessarily and dangerously alienates many peaceful followers of Islam, the world’s second-largest religion.

“I know that a lot of people find appealing the things that Donald says because he says what people wish they could say,” Rubio said. “The problem is presidents can’t just say whatever they want. They have consequences, here and around the world.”

Trump countered that Rubio and other politicians espouse a political correctness and diplomatic tone that endangers Americans.

“Marco talks about consequences,” Trump said. “Well, we’ve had a lot of consequences, including airplanes flying into the World Trade Center, the Pentagon and could have been the White House. . . . You can be politically correct if you want. I don’t want to be politically correct. I want to solve problems. And we have a problem of hate.”

Rubio shot back: “I’m not interested in being politically correct. I’m interested in being correct.”

Kasich said radical Islam is the greatest threat to the United States.

“They want to destroy everything we’re about,” Kasich said. But he also noted that cultivating alliances of shared trust with such Muslim countries as Saudi Arabia and Jordan is critical to the U.S. mission of defeating Islamic State terrorists.

Cruz agreed. “The answer is not simply to yell, ‘China bad, Muslims bad.’ You’ve got to understand the nature of the threats we’re facing and how you deal with them.”

A clear difference emerged on Social Security, with Trump vowing not to tinker with the popular federal retirement program and other candidates arguing that the system requires a sweeping overhaul, including pushing back the retirement age, to avert a future debt crisis.

“I will do everything within my power not to touch Social Security — to leave it the way it is, to make this country rich again, to bring back our jobs, to get rid of deficits, to get rid of waste, fraud and abuse, which is rampant in this country,” Trump said.

Rubio said Trump’s promise is unrealistic. “The numbers don’t add up,” the senator said, a line he repeated again and again.

“The bottom line is we can’t just continue to tiptoe around this and throw out things like, ‘I’m going to get at fraud and abuse,’ ” Rubio said. “You still have hundreds of billions of dollars of deficit that you’re going to have to make up. And here’s the thing: If we do not do it, we will have a debt crisis.”

Cruz sounded a similar call. “Social Security right now is careening towards insolvency, and it’s irresponsible. And any politician that doesn’t step forward and address it is not being a real leader.”

Trump was challenged on the fact that even as he has railed against the effects of international trade and immigration, he has profited from hiring foreign workers and manufacturing clothing in China and Mexico.

“I’m a businessman. These are laws. These are regulations. These are rules. We’re allowed to do it,” Trump said. “So I will take advantage of it; they’re the laws. But I’m the one that knows how to change it. Nobody else on this dais knows how to change it like I do, believe me.”

Cruz and Rubio, meanwhile, stressed the importance of distinguishing between trade deals that help the economy and U.S. workers and those that do not.

“We’re getting killed in international trade right now,” Cruz said. “And we’re getting killed because we have an administration that doesn’t look out for American workers and jobs are going overseas. We’re driving jobs overseas.”

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

Some GOP leaders, including 2012 presidential nominee Mitt Romney, are vowing never to vote for the man who appears increasingly likely to be their party’s standard-bearer in November. There is also more talk of a contested party convention in July, in which GOP leaders might engineer a way of awarding the nomination to someone else.

The billionaire mogul used his opening statement to send a message to those who would stand in his way: Join the movement.

“One of the biggest political events anywhere in the world is happening right now with the Republican Party,” Trump said. “Millions and millions of people are going out to the polls and they’re voting. They’re voting out of enthusiasm. They’re voting out of love. . . . Frankly, the Republican establishment, or whatever you want to call it, should embrace what’s happening.”

Trump also took a more statesmanlike tone when he was asked about violence at his rallies, which has included protesters being roughed up.

“I certainly do not condone that at all,” he said.

“There is some anger,” Trump said of the supporters who show up to cheer him. “There’s also great love for the country. It’s a beautiful thing.”

Kasich, however, suggested that the tone of Trump’s rallies speaks to a larger problem. “I worry about the violence at a rally, period,” the Ohio governor said. “The unity of this country really matters.”

The race will reach what could be an inflection point next week, with primaries in five states, including closely watched Florida and Ohio.

They are must-win for home-state candidates Rubio and Kasich. And for the first time, delegates will be awarded on a winner-take-all basis, which means that if his rivals cannot curb Trump’s momentum, he will accelerate on his path to the nomination.

Thursday’s debate at the University of Miami, sponsored by CNN, The Washington Times and Salem Radio Network, was the last time they would all be on the same stage before the next round of primaries.

A field that numbered nearly 20 candidates in their first face-off in August — so many that the debate had to be split into a main event and an undercard — has shrunk to four.

Trump has led nearly without interruption since then, and he has set the pace and tone, to the dismay of an increasingly impotent Republican establishment.

Rubio tried to put the brakes on Trump, and to get under his skin, by adopting the billionaire’s own tactics. Starting at a debate in Houston on Feb. 25, he unleashed a barrage of personal insults. At one point, he made a joke about the size of Trump’s hands that also suggested his genitalia are small.

But it backfired, as Rubio lost 18 of the next 20 contests.

“At the end of the day, it’s not something I’m entirely proud of,” Rubio acknowledged Wednesday. “My kids were embarrassed by it, and if I had to do it again, I wouldn’t.”

On Thursday night, the adults took a lesson from the kids.

David A. Fahrenthold in Washington contributed to this report.