Here are highlights from CNN's Republican presidential debate on Dec. 15 in less than two minutes. (Sarah Parnass/The Washington Post)

The Republican presidential contenders on Tuesday night framed their final debate of the year around a single question: Which of them is best equipped — by background, tough-mindedness and leadership abilities — to protect the country against terrorism?

Their focus reflected the reality that the nation has been riveted on that issue in the wake of attacks in Paris and San Bernardino, Calif. — the latter carried out by an American citizen and his Pakistani wife, who had been living what appeared to be ordinary lives in their community.

“We have people across this country who are scared to death,” said New Jersey Gov. Chris Christie. “Because I could tell you this, as a former federal prosecutor, if a center for the developmentally disabled in San Bernardino, California, is now a target for terrorists, that means everywhere in America is a target for these terrorists.”

If the mood of the evening was dire to the point of being apocalyptic, the debate also presented the candidates with an opportunity to draw sharp contrasts in their prescriptions for eradicating the threat, whether it comes from the Islamic State abroad or from homegrown enemies.

Celebrity billionaire Donald Trump, the front-runner in the race and the beneficiary of many Republicans’ desire for an anti-establishment outsider, was not the dominant presence he has been in past debates. The other contenders were jockeying against one another as much as the figure who has overshadowed them for the past six months.

From left, Ben Carson, Donald Trump and Ted Cruz at the Republican debate in Las Vegas. (Mike Blake/Reuters)

The candidates had one area of consensus: blaming President Obama’s leadership for the country’s vulnerability and warning that Democratic front-runner Hillary Clinton — who was his first-term secretary of state — would represent a continuation of his policies. They portrayed Obama as indecisive and weak, incapable of generating the respect of America’s allies and the fear of its adversaries.

Where they differed was on what qualities would be most important going forward. Some focused on the specifics of their policies, while others emphasized the depth of their experience or the toughness of character required to be commander in chief.

One of the debate’s sharpest and most substantive clashes was among the three senators on the stage: Marco Rubio of Florida, Ted Cruz of Texas and Rand Paul of Kentucky.

Rubio, one of the more hawkish candidates in the race, criticized Cruz and Paul for their votes that allowed the lapse of the National Security Agency’s program collecting and storing phone metadata.

“This is not just the most capable, it is the most sophisticated terror threat we have ever faced,” Rubio said. “We are now at a time when we need more tools, not less tools. And that tool we lost, the metadata program, was a valuable tool that we no longer have at our disposal.”

The more libertarian Paul, who has repeatedly warned that the program jeopardizes civil liberties, replied that Rubio “gets it completely wrong.”

Both Paul and Cruz zeroed in on an immigration-reform bill championed by Rubio that passed the Senate in 2013 but never got through the House. The immigration issue has become one of the biggest flash points within the Republican Party.

“The one thing that might have stopped San Bernardino, that might have stopped 9/11, would have been stricter controls on those who came here,” Paul said. “And Marco has opposed at every point increased security — border security for those who come to our country.”

Cruz also argued that his Senate colleague “was fighting to grant amnesty and not secure the border. I was fighting to secure the border.”

The senator from Texas said that he does not intend to support granting legal status to the 11 million undocumented immigrants in the United States — the farthest he has gone on a question he has refused to answer. Cruz declined to say what he planned to do with undocumented immigrants in the country now, instead asserting that the border needed to be secured before he did anything about people currently in the country.

“I have never supported legalization, and I do not intend to support legalization,” Cruz said.

As the three senators argued over the details of immigration, federal surveillance and counterterrorism laws, Christie and former Hewlett-Packard chief executive Carly Fiorina stressed instead the importance of executive decision-making experience.

“Listen, I want to talk to the audience at home for a second,” Christie said. “If your eyes are glazing over like mine, this is what it’s like to be on the floor of the United States Senate. I mean, endless debates about how many angels on the head of a pin from people who’ve never had to make a consequential decision in an executive position.”

At another point, Fiorina chimed in to say that voters are “looking for solutions, not lawyers arguing over laws or entertainers throwing out sound bites that draw media attention. We need to solve the problem.”

The debate also brought out a feistier performance from former Florida governor Jeb Bush. Once presumed to be the establishment front-runner by virtue of his famous last name and fundraising power, but now lagging far behind in the polls, Bush launched the fiercest attacks of the evening against Trump’s temperament and leadership style.

“Donald, you know, is great at the one-liners, but he’s a chaos candidate — and he’d be a chaos president,” Bush said. “He would not be the commander in chief we need to keep our country safe.”

Trump, visibly peeved by Bush’s assault, tried to wave off his lower-polling rival. “He has failed in this campaign,” Trump said. “It’s been a total disaster. Nobody cares. And frankly, I’m the most solid person up here.”

The two men clashed again later in the debate. Showing the steel that his supporters have been longing to see, Bush took a verbal swing at Trump and said his proposals to confront the Islamic State reveal his “lack of seriousness.”

Trump responded by mocking Bush as meek: “I think Jeb is a very nice person, but we need toughness. . . . With Jeb’s attitude, we will never be great again. That I can tell you.”

During a third exchange, Bush wondered whether Trump, who has said he gets his foreign policy advice from watching television shows, might watch cartoons on Saturday instead of public affairs programs on Sunday.

Trump himself may have validated Bush’s point at another point in the debate when he appeared not to understand what the “nuclear triad” is. The term refers to the three ways the United States can deliver nuclear weapons: by air, sea or land.

They bickered for so long that other candidates chimed in.

“All the fighting and arguing is not advancing us,” said Ohio Gov. John Kasich. “It is not the way we’re going to strengthen our country. It will strengthen our country when we come together.”

At the end of the debate, Trump ruled out a third-party run.

The debate, which was the fifth for the sprawling field of GOP contenders, came at a phase when the polls are showing a significant shift in their prospects.

Trump continues to top the polls nationally and has even strengthened his lead. In the latest Washington Post-ABC News survey, he garnered 38 percent support among likely Republican voters, which was six points higher than in October and November. He has grown more popular since making the explosive proposal to temporarily ban Muslims from entering the United States.

Meanwhile, Cruz appears to be enjoying a breakout moment, having doubled his support in the past month to be in second place, with 15 percent nationally. More significantly, with the Iowa caucuses just seven weeks away, Cruz is now leading in that first contest of the primary season. The traditionally reliable Des Moines Register poll showed the senator from Texas with a 10-point lead over Trump.

In recent days, Trump has escalated his attacks against Cruz’s demeanor and qualifications to be commander in chief, at one point describing him as a “maniac.” But when co-moderator Dana Bash reminded him of his comments, Trump refused to repeat them.

Trump, standing next to Cruz, reached to pat his rival on the back and said: “He has a wonderful temperament. He is just fine.”

Recent weeks have also seen a slump in the prospects of retired neurosurgeon Ben Carson, in part because of his seeming lack of knowledge and assuredness on foreign policy. His mild manner stands as a contrast with the growing bellicosity of the rhetoric around him.

That became even more evident when he was asked to weigh in on the spirited argument that Rubio and Paul were having on the scope and scale of the role of government in fighting terrorism.

“I think you have to ask them about that,” Carson said. “I don’t want to get in between them. Let them fight.”