Republican U.S. presidential candidate businessman Donald Trump (L) responds to criticism from former Governor Jeb Bush (R) as Senator Ted Cruz (C) looks on during the Republican presidential debate in Las Vegas, Nevada December 15, 2015. [Photo/Agencies]

LAS VEGAS - Republican presidential contender Jeb Bush assailed front-runner Donald Trump in a debate on Tuesday for proposing to ban Muslims from entering the United States, calling Trump a "chaos candidate" who was not serious but adept at delivering one-liners.

But many other candidates, including Florida Senator Marco Rubio and Texas Senator Ted Cruz, were reluctant to directly take on the billionaire Trump, focusing their criticism on President Barack Obama and Democratic front-runner Hillary Clinton, Obama's former secretary of state, for failing to keep the United States safe.

"Donald is great at the one-liners, but he is a chaos candidate and he would be a chaos president," said Bush, the former governor of Florida who has been lagging in opinion polls. He said Trump's Muslim plan was "not a serious proposal."

Trump, 69, dismissed Bush's criticism. "He's failed in his campaign, it's been a total disaster," Trump said, and defended his proposal.

"We're not talking about isolation; we're talking about security. We're not talking about religion; we're talking about security. Our country is out of control," Trump said.

Bush, 62, later criticized Trump for proposing to kill the families of Islamic State militants, saying it was "just crazy" to think that was a solution.

"Donald, you're not going to be able to insult your way to the presidency," Bush said. "Leadership is not about attacking people and disparaging people."