Donald J. Trump came under sustained attack from Jeb Bush and other Republican presidential candidates on Tuesday night as they united against his plan to bar Muslims from entering the United States while tussling over who would be toughest in protecting Americans from terrorist threats.
Senator Marco Rubio of Florida also faced his toughest moments of the race during the latest Republican debate as a top rival, Senator Ted Cruz of Texas, repeatedly questioned his conservative credentials and his judgment on national security and immigration. Though Mr. Rubio at times seemed to gain the upper hand, he looked and sounded rattled as Mr. Cruz portrayed him as lining up with liberals like Senator Chuck Schumer of New York in favoring “amnesty” for immigrants who are in the country illegally.
Mr. Bush, the former Florida governor who is struggling in the polls, was withering as he assailed Mr. Trump’s harsh words and ominous warnings about Muslims and mosques since the mass shootings in Paris and San Bernardino, Calif. While Mr. Trump has managed to channel and stoke the fears of many Americans, Mr. Bush led his rivals in portraying him as a fearmonger more interested in scaring voters than in planning an effective war against the Islamic State.
“Donald, you’re not going to be able to insult your way to the presidency — that’s not going to happen,” Mr. Bush said. “Leadership is not about attacking people and disparaging people. Leadership is about creating a serious strategy.”
Mr. Trump, who has belittled Mr. Bush’s energy and strength since entering the race in June, suggested that his rival was simply pretending to be tough. But his counterattack seemed to backfire, drawing a rebuke from the audience.
“With Jeb’s attitude, we will never be great again, that I can tell you — we will never be great again,” Mr. Trump said, to loud boos.
In the evening’s other major subplot, Mr. Rubio, who has increased his attacks on the surging Mr. Cruz over the last month, engaged his colleague in their most pointed face-to-face confrontation yet. Trying to defuse criticism over his leading role in the 2013 legislation to offer unauthorized immigrants a path to citizenship, Mr. Rubio asserted that Mr. Cruz also wanted to offer legal status to those immigrants.
Mr. Cruz said Mr. Rubio was trying to “muddy the waters” and “raise confusion,” and linked Mr. Rubio to Mr. Schumer and President Obama on the issue.
“I led the fight against his legalization and amnesty” bill, Mr. Cruz said, arguing that to claim his and Mr. Rubio’s records on the issue were the same was “like suggesting the fireman and the arsonist have the same record because they were both at the scene of the fire.”
But when pressed by Mr. Rubio and a moderator on whether he would rule out legalizing undocumented immigrants, Mr. Cruz appeared to leave himself a measure of space.
“I have never supported legalization, and I do not intend to support legalization,” he said.
The exchanges between Mr. Rubio and Mr. Cruz, who have emerged as leading candidates recently, highlighted the party’s most glaring divisions: on immigration, intelligence gathering and foreign intervention.
Mr. Rubio has positioned himself as a Republican who would have been at home in the George W. Bush administration, a hawk on national security but a pragmatist on immigration. (He has, though, backed off his support for a comprehensive immigration overhaul in the face of scathing criticism from the right.)
Mr. Cruz, by contrast, has tried to run as a post-Bush Republican, taking a hard line on immigration while seeking a middle ground between the party’s interventionists and libertarians on defense issues.
After Mr. Rubio tried to portray him as soft on national security, Mr. Cruz linked him to Democratic foreign policy on crises like Libya.
“One of the problems with Marco’s foreign policy is he has far too often supported Hillary Clinton and Barack Obama undermining governments in the Middle East that have helped radical Islamic terrorists,” Mr. Cruz said. “We need to focus on killing the bad guys, not getting stuck in Middle Eastern civil wars that don’t keep America safe.”
Mr. Rubio shot back that while Mr. Obama and Mrs. Clinton wanted to “lead from behind,” Mr. Cruz was suggesting “not to lead at all.”
Safety and fear have not loomed so powerfully over a debate, or an electorate, since the Sept. 11, 2001, attacks. But if the threat of terrorism has become the defining issue in the race, Republicans are sharply divided on the toughest and smartest strategies to prevent more attacks.
The candidates used words like “angry” and “betrayed” as they described an America besieged by terrorist threats and exposed and vulnerable after seven years of Mr. Obama’s leadership, which they argued Mrs. Clinton, who polls show is leading the Democratic race, would only continue.
The campaign’s new focus on terrorism was on display when Mr. Cruz was questioned about his vote to halt the National Security Agency’s ability to collect bulk phone data. He framed the measure, which ended elements of the Patriot Act, as a “reform of how we target bad guys” and argued that it had actually helped fight terrorism.
Mr. Rubio, though, kept the focus on ending the bulk data collection, arguing that Mr. Cruz had given away “a valuable tool” against terrorism.
“I promise you, the next time there is 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?” Mr. Rubio said. “And the answer better not be, ‘Because we didn’t have access to records or information that would have allowed us to identify these killers before they attack.’ ”
Senator Rand Paul of Kentucky, though, came to Mr. Cruz’s defense with a fierce attack on Mr. Rubio. Mr. Paul, who has sought to revive his languishing campaign by reasserting his libertarianism, mocked Mr. Rubio for proclaiming himself “great and strong on national defense” when, Mr. Paul said, he was “the weakest of all the candidates on immigration.”
“He has more of an allegiance to Chuck Schumer and the liberals than he does on conservative policy,” Mr. Paul said, referring to Mr. Rubio’s work with Mr. Schumer on an immigration overhaul.
Forced to respond to Mr. Trump’s proposal to bar Muslims from America, Mr. Cruz — who is ahead of Mr. Trump in Iowa in one poll — and Mr. Rubio, who has also been gaining strength, only briefly registered their opposition before focusing their fire on Mr. Obama.
Gov. Chris Christie of New Jersey, enjoying a measure of good timing, won applause at the Las Vegas debate after the bickering among Mr. Cruz, Mr. Paul and Mr. Rubio by disparaging their place of employment and emphasizing his executive experience.
“This is the difference between having been a federal prosecutor and actually doing something, and just spending your life as one of 100 debating it,” he said. “Let’s talk about how we do this and not about which bill these guys like more. People don’t care about that.”
Mr. Christie repeatedly assailed Mr. Obama and Mrs. Clinton, saying they had understated the threat from Islamic State terrorists, and argued for more muscular intelligence-gathering tools to “protect the safety and security of Americans.”
But his judgment came under a rare attack from a fellow Republican for his aides’ closing of entrances to the George Washington Bridge. “When we think about the judgment of someone who might want World War III, we might think about someone who might shut down a bridge,” Mr. Paul said.
Gov. John R. Kasich of Ohio, too, called for more aggressive government surveillance of suspected terrorists. Mr. Bush, while also urging strong law enforcement actions, endorsed the sentiment of President George W. Bush, who said “Islam is peace” after the Sept. 11 attacks.
“We can’t dissociate ourselves from peace-loving Muslims,” Mr. Bush said, responding to a moderator’s question. Reiterating his months-old plan against the Islamic State, he said: “It requires leadership. It’s not filing an amendment and call it a success. It’s about developing a strategy, leading the world.”
Negative exchanges — in word choice, tone, put-downs and facial expressions — dominated the debate, the fifth and final one of the year for the Republicans. The contentiousness reflected an intensely competitive race and a widely held belief among the candidates that anxiety-ridden voters are looking for the fiercest possible nominee — not only against the Islamic State, but also against the likely Democratic candidate, Mrs. Clinton.
The assault on Mr. Trump began in the debate’s opening statements, with Mr. Paul comparing him to totalitarian Chinese officials because he supports monitoring the Internet for suspicious activity. Mr. Bush quickly picked up the theme, saying Mr. Trump was not a serious candidate but merely “great at the one-liners.”
“He is a chaos candidate; he would be a chaos president,” Mr. Bush said. “He would not be the commander in chief we need to keep our country safe.”
Mr. Trump dismissed Mr. Bush’s attacks as the flailing of a fading candidate.
“Jeb doesn’t really believe I’m unhinged,” he said. “He said that very simply because he has failed in this campaign. It’s been a total disaster. Nobody cares.”
Mr. Bush, who grew tougher as the night went on, responded repeatedly that Mr. Trump was not a serious candidate. “Banning all Muslims will make it harder for us to do exactly what we need to do, which is to destroy ISIS,” he said.
Later, Mr. Bush delivered one of the most biting lines of the night, alluding to a past remark by Mr. Trump about where he got his military advice. “I won’t get my information from the shows,” Mr. Bush said. “I don’t know if that’s Saturday morning or Sunday morning.”
Any expectation that Mr. Cruz and Mr. Trump would clash was snuffed out when Mr. Trump was read back his criticism of Mr. Cruz from last weekend — when he called Mr. Cruz “a maniac” — and retreated from it.
“He’s just fine,” Mr. Trump said, reaching over to pat Mr. Cruz on the back. “Don’t worry about it.”
Earlier, Mr. Cruz engaged Mr. Trump in a bit of bonhomie, joking that he would build a border wall, “and I’ll get Donald Trump to pay for it.”
“I’ll do it!” Mr. Trump responded.
Mr. Trump, after suggesting in recent weeks that he might abandon the party, appeared to reiterate his pledge from earlier in the year that he would not run as a third-party candidate.
And in an interview on CNN after the debate, Mr. Trump was unambiguous. “Yes, I’m a Republican, and I’m going to be a Republican,” he said. “I’m not going to be doing a third-party.”
Ben Carson, the retired neurosurgeon who enjoyed a burst of popularity this fall but has fallen in polls amid questions about his lack of national security experience, salted his remarks with statistics and other evidence of greater preparation. But he was otherwise a low-key presence in the debate, and complained that a “false narrative” had emerged about his readiness to lead the country.
“I have a lot of experience building things, organizing things — a national scholarship program,” Mr. Carson said. “Some people say, ‘You’re weak because you’re not loud and you’re not boisterous and you’re not rude.’ But the fact of the matter is, look and see what I’ve done, and that speaks volumes about strength.”