Cruz, Rubio clash in GOP debate as Trump defends call to bar Muslims

In a presidential race reshaped by national security concerns, Republican front-runner Donald Trump defended his provocative call for a total bar on Muslims from the United States as the other candidates pushed their own plans for fighting the Islamic State in Iraq and the Levant (ISIL).

The debate, with its focus on security, reflected how the recent attacks in Paris and San Bernardino, California, have changed the race. Hours before the debate began, officials in Los Angeles closed all schools after an emailed threat that was later deemed a hoax.

The attacks have ignited a political debate about President Barack Obama's campaign to defeat ISIL in the Middle East and the nation's security posture in preventing attacks in the U.S.

The debate comes at a time when Texas Sen. Ted Cruz is on the rise, challenging Trump's months-long lead in Iowa, whose Feb. 1 caucuses lead off the state-by-state nominating contests.

Cruz is trying to pitch himself as a more electable alternative to Trump, but some Republican leaders believe his hard-line conservative positions and prickly demeanor would put him at a disadvantage in a general election contest against Democratic front-runner Hillary Clinton. Trump, as if to ease that worry, promised anew he'd stick with the GOP, saying he'll pass on running as a third-party or independent candidate.

Florida Sen. Marco Rubio has also entered the top tier in the crowded field by seeking to straddle the divide between his party's establishment and more conservative wings.

Trump insisted his proposal to restrict Muslims entry to the U.S., which was roundly condemned by his rivals, wasn't an attempt to discriminate.

“We are not talking about isolation, we're talking about security,” he said. “We are not talking about religion, we are talking about security.”

Former Florida Gov. Jeb Bush found his footing and dismissed Trump's proposal as unserious, saying, “Donald is great at the one-liners, but he's a chaos candidate and he'd be a chaos president.” The audience “oohed” at the jab, and Trump responded that Bush was only attacking him “because I'm leading and his campaign has been a total disaster and nobody cares.”

In contrast, Cruz and Rubio said they understood why Trump had raised the idea and avoided directly criticizing the front-runner.

Instead, they went after each other, engaging in lengthy debates over their differences on national security and immigration, one of the most contentious issues in the Republican primary.

Rubio defended his support for eventually providing a pathway to citizenship for some people in the U.S. illegally, an unpopular position in the Republican Party. He was a co-author of comprehensive bipartisan Senate legislation in 2013 that would have created that pathway, but he has since said the nation's immigration crisis must be addressed in piecemeal fashion, with legalization only an option after the U.S.-Mexico border is secured.

Cruz went further than he has previously in opposing legalization for people in the U.S. illegally, declaring, “I have never supported legalization and I do not intend to support legalization.”

The senators — both Cuban-Americans in their 40s — have been sparring from afar for weeks.

Rubio positioned himself as the hawk on national security, defending American efforts to oust dictators like Syria's Bashar al-Assad from the Middle East. He also accused Cruz of weakening the government's ability to track terrorists because he voted in favor of legislation to eliminate the National Security Agency's bulk phone-records collection program and replace it with a more restrictive effort to keep the records in phone companies' hands.

Cruz argued that his vote helped “reform how we target bad guys” by allowing the government to search more phone numbers to uncover terrorists.

Beginning after the Sept. 11, 2001 attacks, the NSA secretly collected the daily calling records — but not contents of conversations — for most Americans, including people never suspected of any crime.

A new law, called the USA Freedom Act, passed in June with broad, bipartisan support. It ordered the NSA to end bulk collection after a six-month transition that expired last week.

The senators also displayed differences in their strategies for targeting the Islamic State. The extremist group claimed responsibility for the Nov. 13 attacks in Paris, and one of the shooters in California pledged allegiance to the group on Facebook shortly before she and her husband shot and killed 14 people at a holiday party.

Cruz called for using “overwhelming air power” to destroy ISIL, while Rubio said airstrikes would have to be supplemented by ground troops, including American special operations forces.

Obama has largely relied on airstrikes to target the ISIL in Iraq and Syria. However, he's also sent troops to Iraq to help train and assist local forces and recently approved sending special operations forces into Syria.

New Jersey Sen. Chris Christie, who has been on the rise in New Hampshire whose primary follows the Iowa caucuses, sought to assert himself in the national security discussions. Christie frequently addressed the cameras directly — a different strategy than most of the others. He also repeatedly referenced his time as a federal prosecutor, saying he was the only one who'd dealt with issues related to terrorism.

Christie called for a no-fly zone over Syria and vowed to shoot down a Russian plane if it were to violate that space.

“Not only would I be prepared to do it, I would do it,” he said. “We would shoot down the planes of Russian pilots if, in fact, they were stupid enough to think that this president was the same feckless weakling that the president we have in the Oval Office is right now.”

The debate's focus on national security was a detriment for retired neurosurgeon Ben Carson, who has struggled on complex international matters. He punted on questions about surveillance and his own qualifications for being commander in chief. His campaign is on the decline after surging in recent months.

Also on the main stage Tuesday night were former business executive Carly Fiorina; Ohio Gov. John Kasich and Kentucky Sen. Rand Paul.

Fiorina said the country needs “someone who's made tough calls in tough times” as a commander-in-chief instead of first-term senators “who never made executive decisions in their life.”

She also said she wants to bring back a “warrior class” of generals, including David Petraeus and Stanley McChrystal, who she said retired early because they told Obama things he didn't want to hear.