The nine leading Republican candidates for president spoke in dire terms Tuesday of the threat of terrorism, as they jockeyed during a televised debate to project the strength and smarts needed to defeat the Islamic State and keep the nation safe.

Front-runner Donald Trump defended his proposal to ban Muslims from traveling to the United States, and repeated his call to kill the family members of Islamic terrorists.

"That will make people think," Trump said. "They may not care about their own lives, but, believe it or not, they care about their families' lives."

An aggressive Jeb Bush portrayed Trump as an unserious candidate who has admitted he gets foreign policy advice from Sunday political talk shows, and who would alienate the very Arab allies the U.S. needs to help take down ISIS.

"He's a chaos candidate and he'd be a chaos president," said Bush, a former governor of Florida, whose campaign has failed to gain much traction.

"I think Jeb is a very nice person, but we need tough people," Trump said.

Bush interrupted him. "Donald, you're not going to be able to insult your way to the presidency," he said as the real estate developer tried to interject. "A little of your own medicine," Bush added.

It was the first debate since the ISIS-inspired terrorist attacks in Paris and San Bernardino, Calif., which sparked even more concern over national security in a race that has been dominated by the subject.

In skirmishing over who would best keep America safe, the candidates often talked over one another and badgered the moderator for more time.

Trump repeated a pledge to "close" parts of the Internet, in an effort to keep ISIS from using it as a recruiting tool. "I don't want them using our Internet to take our young, impressionable youth," he said.

Kentucky Sen. Rand Paul said Trump's idea would violate the First Amendment guarantee of freedom of speech.

"Is Donald Trump a serious candidate?" Paul said. Killing the families of terrorists would violate the Geneva Convention, he argued, and "defy every norm that is America." Would-be Trump supporters need to ask themselves whether they believe in the Constitution, Paul said.

"So they can kill us, but we can't kill them?" Trump asked.

In just 47 days, the Republican nominating contest starts with the Iowa caucuses, and the candidates sought momentum for the final postholiday push to the voting.

Texas Sen. Ted Cruz has emerged to challenge Trump for front-runner status in several recent Iowa polls, though Trump remains the national front-runner. They were expected to clash but largely avoided direct confrontation in the two-hour debate.

Before the debate, the nonaggression pact between them had begun to crack. In a private fund-raiser last week, Cruz said it was a "challenging question" whether Trump has the "judgment" to be president, according to a leaked recording of the event. Trump called Cruz a "bit of a maniac."

Cruz and Florida Sen. Marco Rubio, a favorite of the GOP establishment who also has appeal to social conservatives, scrapped over their records.

Rubio blasted Cruz for supporting a measure, signed into law by President Obama, that ended the National Security Agency's bulk collection of metadata on Americans' telephone calls - which Rubio said was a key tool in detecting terrorist communications.

Cruz ripped Rubio's earlier support of proposed legislation that would have liberalized immigration laws, including allowing a path to citizenship for some of those illegally in the U.S. Cruz said he "led the fight against" Rubio's "legalization and amnesty bill."

Gov. Christie, who has recently gained momentum in New Hampshire, dismissed their heated back and forth.

"If your eyes are glazing over like mine, this is what it's like to be on the floor of the United States Senate," Christie said. "I mean, endless debates about how many angels on the head of a pin from people who have never had to make a consequential decision in an executive position."

Christie said he has to be accountable as governor and prosecuted terrorism cases as U.S. attorney in New Jersey.

The candidates also debated whether to implement and enforce a no-fly zone in Syria, where ISIS controls large swaths of territory. Christie supports the idea.

Asked if he would shoot down Russian planes that invaded air space in a no-fly zone, Christie said he would, adding he wouldn't be a "feckless weakling" like Obama.

Paul, turning to Christie, responded, "If you're in favor of World War III, you have your candidate."

The candidates clashed sharply over whether regime change in Syria should be an aim of U.S. policy. Cruz, Paul, and Trump argued against regime change, while Rubio, Christie, and Ohio Gov. John Kasich advocated toppling Syria's president, Bashar al-Assad.

"If we topple Assad, ISIS will take over Syria," Cruz said. The U.S. should focus on defeating ISIS, he said.

Rubio responded, "We will have to work around the world with less-than-ideal governments," such as Saudi Arabia's. "But anti-American dictators like Assad," Rubio said, "if they go I will not shed a tear."

Asked broadly about regime change in Iraq and elsewhere, Trump called for more investment at home on things like infrastructure.

"Wow, that is exactly what President Obama has said," former technology executive Carly Fiorina responded.

Paul said regime change in Iraq and Libya has unleashed chaos. "From the chaos you've seen repeatedly the rise of radical Islam," he said.

As Trump has continued to soar, party leaders have started to worry that were he the nominee, the Republicans would suffer a landslide loss in 2016 that would crush the party's down-ballot candidates and, at a minimum, cost control of the Senate.

Trump recently revived his threat to run as an independent if he isn't treated "fairly" by the GOP, but in answer to a question in the debate Tuesday he said he was "totally committed to the Republican Party."

Trump has not been damaged by a series of incendiary comments about Mexicans, illegal immigrants, women and others.

Indeed, about six in 10 Republicans endorse the idea of banning Muslim immigration, according to a Washington Post-ABC News Poll released Tuesday.

That survey showed Trump's lead was larger than ever - 38 percent among registered Republicans and GOP-leaning independents. It was based on interviews conducted entirely after Trump's Dec. 7 ban proposal.

Retired neurosurgeon Ben Carson was challenging Trump at the top of national polls earlier this fall, but his prospects have faded amid stumbles on foreign policy.

215-854-2718@tomfitzgerald