Trump defends call for banning Muslims from U.S.

Republican front-runner Donald Trump defended his provocative call for banning Muslims from the United States in Tuesday night's presidential debate, but his plan was brushed aside as unserious by his rivals as they outlined their own approaches for preventing attacks in the United States.

"Donald is great at the one-liners, but he's a chaos candidate and he'd be a chaos president," former Florida Gov. Jeb Bush said as the fifth Republican debate opened in Las Vegas.

Florida Sen. Marco Rubio said of Trump's proposal, "It isn't going to happen."

Trump insisted 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."

Tuesday's debate was the first for Republicans since the attacks in Paris and San Bernardino, California, that increased concerns about terrorism in the United States. Hours before the debate was to begin, officials in Los Angeles closed all schools after an emailed threat that was later deemed a hoax.

Security fears have reshaped the presidential race, though outsiders like Trump continue to dominate the GOP contest. The billionaire is facing a new challenge from Texas Sen. Ted Cruz, who has been rising in polls.

Cruz, along with Rubio, said he understood why Trump had made his proposal for banning Muslims, and he avoided directly criticizing the front-runner. Cruz said he believes a more narrowly focused approach would more effectively target Islamic militants.

"It's not a war on a faith," he said.