LAS VEGAS — In a presidential race reshaped by national security fears, Sens. Ted Cruz and Marco Rubio clashed over U.S. military intervention, government spying on Americans’ communications and immigration Tuesday night, as front-runner Donald Trump defended his provocative call for banning Muslims from the United States.

Struggling former Florida Gov. Jeb Bush found his footing in trying to discredit Trump’s qualifications for the White House, chiding the brash billionaire for trying to “insult your way to the presidency.”

Tuesday night’s debate was the first for Republicans since the attacks in Paris and San Bernardino, California, which heightened fears of terrorism in the United States. The attacks have ignited a political debate about President Barack Obama’s campaign to defeat the Islamic State in the Middle East and the nation’s security posture in preventing attacks in the U.S.

Trump’s call for temporarily banning Muslims from the U.S. — a proposal roundly criticized by his rivals — dominated much of the discussion heading into the debate. He said he wasn’t seeking to discriminate against Muslims.

“We are not talking about isolation; we’re talking about security,” he said. “We are not talking about religion, we are talking about security.”

Bush dismissed the proposal as unserious, saying, “Donald is great at the one-liners, but he’s a chaos candidate and he’d be a chaos president.”

In a moment that might help ease anxiety among Republican leaders, Trump pledged he would not seek to run as an independent. If he should lose the nomination, some fear he would make such a move, possibly preventing the nominee from defeating the Democratic challenger. “I am totally committed to the Republican Party,” Trump said.

He was largely spared from criticism by Cruz and Rubio, who said they understood why Trump had raised the idea of banning Muslims. Instead, they focused on each other, engaging in lengthy debates over their differences on national security and immigration, one of the most contentious issues in the Republican primary.

Rubio, of Florida, defended his support for eventually providing a pathway to citizenship for some people in the U.S. illegally, an unpopular position within the Republican Party. Rubio was a co-author of comprehensive Senate legislation in 2013 that would have created that pathway, but he has since said the nation’s immigration crisis must be addressed in piecemeal fashion, with legalization only an option after the U.S.-Mexico border is secured.

Seeking to draw a sharp contrast with Rubio, Cruz went further than he has previously in opposing legalization for people in the U.S. illegally. He declared, “I have never supported legalization and I do not intend to support legalization.”

The two senators — both Cuban-Americans in their 40s — have been sparring from afar for weeks, and their rivalry could become one of the dominant forces in the race as the first voting contests in February draw near. The Texas senator is on the rise, particularly in Iowa’s kickoff caucuses, and is casting himself as a more electable alternative to Trump, while Rubio is seeking to straddle the divide between his party’s establishment and more conservative wings.

Rubio positioned himself as the hawk on national security, defending American efforts to oust dictators like Syria’s Bashar Assad from the Middle East. He also accused Cruz of weakening the government’s ability to track terrorists because he voted in favor of legislation to eliminate the National Security Agency’s bulk phone-records collection program and replace it with a more restrictive effort to keep the records in phone companies’ hands.