Las Vegas — Personal attacks, withering ripostes and an unexpected declaration of party unity marked the last Republican debate of 2015 Tuesday as competing, sharp-toned proposals on combating terrorism signaled urgent efforts to break out of the pack six weeks before the first contest of the 2016 presidential race.

The most contentious debate of the season ended with front- runner Donald Trump reiterating his pledge to forego an independent bid for the presidency if he fails to win the nomination.

"I am totally committed to the Republican party. I feel very honored to be the front runner," he said. "I will do everything in my power to beat Hillary Clinton."

Trump, who has made that pledge once before but subsequently waffled on it, made his statement at the end of a debate in which he joined President Barack Obama and Democratic front-runner Hillary Clinton as targets of fierce attacks at a televised debate.

The fifth Republican debate marked the first time the candidates have met since two deadly terrorist attacks in Paris on Nov. 13 and in San Bernardino, California on Dec. 2 and it offered the starkest-yet display of the split between the party's hawks and its more libertarian-leaning candidates over American security at home and abroad.

There were sharp disagreements on immigration and refugee policy and how to combat terrorism overseas.

In one particularly sharp exchange,New Jersey Gov. Chris Christie called President  Obama a "feckless weakling" and promised to shoot down Russian planes "If you are in favor of World War III," retorted Sen. Rand Paul of Kentucky, "you have your candidate."

The first attacks of the incendiary CNN/Facebook debate, held at the Venetian casino in Las Vegas, focused on the front- runner.

"Donald is great at the one-liners, but he is a chaos candidate and he'd be a chaos president," former Florida Gov. Jeb Bush said. "He would not be the commander in chief we need to keep our country safe." Later as the two tangled over airtime, Bush snapped at Trump: "You're not going to be able to insult your way to the presidency."

With less than two months remaining before the Feb. 1 Iowa caucuses and the Feb. 9 New Hampshire primary, other candidates quickly muscled their way into the scrum and the prime-time limelight. Two particular confrontations dominated the initial hour of the main debate: Trump versus Bush and one between two Cuban-American freshmen senators: Ted Cruz of Texas and Marco Rubio of Florida.

But the expected confrontation between Trump and Cruz, who has overtaken the billionaire in Iowa polls, never occurred. "He has a wonderful temperament," said Trump, patting Cruz on the shoulder after shrugging off a question about why, earlier in the week, he had called the senator "a maniac."

Rubio attacked Cruz, who has been surging in recent polls, over a variety of issues, criticizing the Texan for voting against spending bills that included military funding. "You can’t carpet bomb ISIS if you don’t have planes and bombs to attack them with," he said.

Rubio also attacked Cruz for his vote earlier this year in favor of a law that put restrictions on how much phone data the National Security Agency can gather without a warrant, saying that Cruz "took away a tool" that allowed law enforcement "to know who terrorists have been calling."

Cruz pointed to Rubio's support of a failed bipartisan immigration bill two years ago that would have offered undocumented immigrants a path to citizenship.

"He was fighting to grant amnesty, and not to secure the border," Cruz said. "I was fighting to secure the border."

Paul, whose lagging poll numbers made his presence on the prime-time debate stage doubtful until the last minute, also attacked Rubio over the immigration."Marco can't have it both ways," said the Kentuckian, complaining about Rubio's refusal to support restrictions on refugees. "He's the weakest of all the candidates on immigration."

Former Hewlett-Packard CEO Carly Fiorina knocked Cruz and Rubio as not being ready to be a commander in chief, saying the nation needs someone with executive experience and "not first-term senators who have never made an executive decision in their life."

Christie piled on the senators, sarcastically commiserating with viewers after one of their exchanges. "If your eyes are glazed over like mine, this is what it's like on the floor of U.S. Senate," he said.

Trump called for a ban on Muslim travel to the U.S. five days after the San Bernardino killing spree, perpetrated by a married couple who were inspired by the Islamic State, investigators have found. All the other Republican candidates, with the exception of Cruz, have sharply criticized his proposal.

Paul accused the billionaire of embracing un-American ideals, such as wanting to "turn off that Internet thing."

Trump later defended himself, saying that the Islamic State "is using the Internet better than we use the Internet."

"We should be using our most brilliant minds to figure out a way that ISIS cannot use the Internet," he said.

On his proposal to ban Muslim travel to the U.S. he was also unapologetic."We've opened up a very big discussion that needed to be opened up," he said to applause from the audience.

Ben Carson said "we need to start thinking about the needs of the American people before we go and try to solve everybody else's problems. ... The fact of the matter is the Middle East has been in turmoil for thousands of years. For us to think that we're going to go in there and fix that with a couple of little bombs and a few little declarations is relatively foolish."

In the undercard debate earlier in the evening, four lower-polling candidates expressed feelings ranging from concern to outrage about the billionaire's plan to ban Muslims from traveling to the U.S.

New York Gov. George Pataki called Trump "the know- nothing candidate of the 21st century," referring to a 19th century movement that targeted Catholic immigrants. He and Sen. Lindsey Graham of South Carolina questioned Trump's fitness to be the nation's commander in chief.

"Declaring war on the religion only helps ISIL," said Graham of the Islamic State. "It's the worst possible thing you can do in this war," he added, saying Trump's proposed travel ban "has made us less safe."

Afterwards, Graham told reporters that he was so upset by the tone of some of his rivals he was nearly in tears on stage. "Some of these foreign policy discussions are hurting us," he said.

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