Republican presidential contender Jeb Bush has sharply criticised frontrunner Donald Trump in a debate for proposing to ban Muslims from entering the United States, calling him a "chaos candidate" adept at delivering one-liners.
But candidates Marco Rubio and Ted Cruz refused to bash Trump on Tuesday for his controversial proposal, focusing their criticism on President Barack Obama and accusing him of failing to keep the US 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 dismissed Bush's criticism. "He's failed in his campaign, it's been a total disaster," he 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.
Trump has dominated the past few weeks of the campaign with his call for a total ban on Muslims entering the US, following a married couple's December 2 massacre of 14 people in San Bernardino, California, inspired by the Islamic State of Iraq and the Levant (ISIL) group.
Most of Trump's Republican rivals, as well as officials and leaders in the US and around the globe, have criticised Trump's proposal although many of his supporters have voiced sympathy with his views.
With seven weeks to go before the first nominating contest in Iowa, Trump, 69, has held or expanded his lead in national polls in the Republican race for the November 2016 presidential election. Cruz, 44, has moved past him in some Iowa surveys.
The Republican debate on Tuesday was the first since the attack in California and another in Paris, and was expected to focus heavily on national security.
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The latest Reuters/Ipsos poll, completed after Trump's December 7 call for a ban on Muslims, showed him leading the field with the support of 33 percent of Republican voters.
Cruz was second at 15 percent, followed by retired neurosurgeon Ben Carson at 12 percent, Marco Rubio, a US senator from Florida, at 10 percent, and former Florida Governor Jeb Bush at 9 percent.
Also appearing in the main debate were New Jersey Governor Chris Christie, Ohio Governor John Kasich, US Senator Rand Paul of Kentucky, and former corporate executive Carly Fiorina.
At the so-called "undercard" debate of low-polling candidates, held before the main event, former New York Governor George Pataki blasted Trump's Muslim proposal as "un-American, unconstitutional, and it is wrong".
Lindsey Graham, a US senator from South Carolina with hawkish views on national security, was especially withering.
"Donald Trump has done the one single thing you can't do: Declare war on Islam itself," Graham said, calling the proposal a coup for ISIL fighters trying to recruit new members.