LAS VEGAS (AP) — Sens. Ted Cruz and Marco Rubio clashed sharply over national security and immigration in Tuesday’s Republican presidential debate, thrusting their evolving feud to the forefront of the GOP race. Front-runner Donald Trump stood firmly behind his provocative call for banning Muslims from the United States, saying, “We are not talking about religion, we are talking about security.”

For former Florida Gov. Jeb Bush, the fifth GOP debate was an opportunity to find his footing after months of uneven performances. He appeared more comfortable than in previous debates in taking on Trump, though it’s unclear whether his stronger showing will change the trajectory of his sluggish campaign.

The prime-time 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 through the debate and into the next morning. “He doesn’t have a plan, he’s not a serious candidate,” Bush said today, speaking on MSNBC.

“The idea that you can just prey on people’s fears is not who we are as a nation, not who we are as a party.”

In a moment that might help ease anxiety among Republican leaders, Trump pledged he would not 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.