Republican presidential candidates Ben Carson, left, Donald Trump, center, and...

Republican presidential candidates Ben Carson, left, Donald Trump, center, and U.S. Sen. Ted Cruz (R-Texas) stand on stage during the CNN Republican presidential debate on December 15, 2015 in Las Vegas, Nevada. Credit: Getty Images / Justin Sullivan

The final Republican presidential debate of the year, focused on terrorism and national security, featured strategic attacks by candidates aiming to knock down, or even knock out, rivals in their way just weeks before the first caucus.

The top clashes pitted Jeb Bush versus Donald Trump and Marco Rubio versus Ted Cruz.

There was also Chris Christie, calling himself a “plain language” New Jersey guy, versus anyone with Washington ties as the candidates angled for support among GOP primary voters, and Rand Paul railing against “regime building” foreign strategies and “bulk collection” of Americans’ private information.

Insults, attacks and policy critiques were all part of the mix as the candidates squared off in Las Vegas.

“Donald is great at the one-liners, but he’s a chaos candidate, and he’d be a chaos president,” Bush, the former Florida governor, said, calling Trump’s idea after the San Bernardino terrorist attack to block all Muslims from entering the U.S. not serious. That set off one of the most strident exchanges.

Trump said Bush’s candidacy was a “total disaster.” At one point, he boasted: “I’m at 42 and you’re at 3,” referring to each candidate’s best and worst standing in the national polls.

“Jeb is a very nice person. But we need toughness,” Trump said.

“Donald, you are not going to be able to insult your way to the presidency. And I do have strength,” Bush countered.

“Oh, you’re tough Jeb,” Trump said dismissively. (Near the close, Trump withdrew his threat to run as an independent, saying he’d support whoever is the GOP candidate.)

Paul, the Kentucky senator trailing badly in the polls, looked to gain by saying Trump’s ideas of blocking refugees and shutting down parts of the Internet would prove not only ineffective but unconstitutional.

But overall, Trump faced few other direct attacks — which, notably, didn’t allow him rebuttal time like in other debates where he had some of the most airtime. Through long early segments of the debate, he was not heard from.

This time, Rubio and Cruz, senators from Florida and Texas respectively, got the most airtime, according to National Public Radio — a shift that perhaps reflected race trends.

Rubio sought to pivot every question to an attack on President Barack Obama or Democratic front-runner Hillary Clinton. Rubio said he supports a pathway to citizenship for illegal immigrants already in the U.S.

He got into it with Cruz over the use of military force to fight ISIS and Islamic terrorists and over domestic surveillance.

Cruz — who had surged to the lead in Iowa, site of the first caucus on Feb. 1, in recent polls — said he was deploy “carpet bombing” to “utterly destroy” ISIS. But pressed by moderators whether he would carpet bomb Raqqa, the ISIS stronghold that functions as its capital with hundreds of thousands of civilians, Cruz said: “The object isn’t a city.”

“They can’t just be defeated by airstrikes,” Rubio said, adding that ground troops with American allies would be necessary.

Cruz defended his vote to end the federal government’s bulk collection of “phone metadata of millions of law-abiding citizens” because it was overreaching.

Rubio said it took “away a valuable tool.”

“Because I promise you, the next time there is attack on — an attack on this country, the first thing people are going to want to know is, why didn’t we know about it and why didn’t we stop it?” Rubio said.

The back-and-forth opened the door for Christie, who looked at the camera as if addressing the Average Joe.

“If you’re eyes are glazed over like mine, this is what it’s like to be on the floor of the United States Senate,” Christie said.

He sought to indirectly tag Cruz, Rubio and others as not doing anything while they were in Congress to reduce terrorism.

“They talk like they were bystanders,” Christie said, whom many observers think is angling not for Iowa, where evangelicals are a key element of the caucus vote, but New Hampshire where independents can vote in the party primaries.

Paul also took a shot at Rubio on national security: “He is the weakest of all candidates on immigration ... has more allegiance to Chuck Schumer and the liberals.”

Paul hit Christie for saying he would shoot down Russian planes if they crossed into a no-fly zone some favor for Syria. “I think when we think about the judgment of someone who might want to start World War III, we might think about someone who might shut down a bridge because they don’t like their friends.”

Former Hewlett-Packard CEO Carly Fiorina invoked late British Prime Minister Margaret Thatcher in saying a woman could be a tough commander in chief and took a veiled shot at Cruz and Rubio.

“To wage war, we need a commander in chief who has made tough calls in tough times and stood up to be held accountable over and over, not first-term senators who’ve never made an executive decision in their life,” Fiorina said.

Neurosurgeon Ben Carson, who led in Iowa until advisers criticized his grasp of foreign policy in a news story, was asked if he had the resolve to wage war, which could kill children.

Carson said he’s made tough calls, such as telling children he needed to operate on their brains.

“Ruthless is not the necessarily the word I would use, but tough ... to what is necessary to get it done,” Carson said.

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