Donald Trump (L) reaches out to Sen. Ted Cruz the Republican presidential debate in Las Vegas, Dec. 15

LAS VEGAS — Challenger Jeb Bush locked horns with Republican front-runner Donald Trump in a national security debate on Tuesday but most rivals avoided tangling with the billionaire who has emerged as an enduring force in the 2016 U.S. presidential campaign.

Their reticence was a nod to Trump’s ability to withstand attacks and return fire. Last week, Trump stunned the field by proposing to ban Muslims from entering the United States, a move his rivals assailed only to find many Republican voters backed the idea and Trump’s lead in opinion polls grew.

The Republicans’ last debate of the year was a lively affair in a city famed for heavyweight boxing matches. Although Bush assailed Trump as never before, it was perhaps too little too late to save his struggling campaign.

“Donald is great at the one-liners, but he is a chaos candidate and he would be a chaos president,” Bush, 62, said.

Trump, 69, shot back: “Oh yeah, and you’re a tough guy, Bush,” noting Bush’s falling standing in the polls and his resulting move down toward the end of the debate stage.

But most of the other seven candidates on stage notably refused to take Trump on.

Voicing appreciation for the response he had received on the campaign trail, Trump seemingly ended any further speculation that he might bolt from the Republican Party and run as an independent candidate, which would almost certainly mean a Democrat would win the White House in the Nov. 8 election.

“I am totally committed to the Republican Party. I feel very honored to be the front-runner,” Trump said.

There was no candidate more eager to avoid riling Trump than Ted Cruz, the U.S. senator from Texas who The New York Times said had criticized Trump at a private fundraiser last week in New York by questioning whether the real estate mogul had the judgment to be in control of the U.S. nuclear arsenal.

With Cruz’s criticism on an audio recording, there was an expectation that Trump and Cruz would finally set aside their truce, especially after Cruz had seized the lead in a major poll this week of Republican voters in Iowa, the state which on Feb. 1 holds the first nominating contest of the election.

Given the chance by CNN moderators to mix it up with the former reality TV star, Cruz demurred.

“What I said in private is exactly what I’ll say here, which is that the judgment that every voter is making of everyone of us up here is who has the experience, who has the vision, who has the judgment to be commander in chief,” Cruz said.

Later, Trump slapped Cruz on the back in appreciation.

“He’s just fine. Don’t worry about it,” Trump said when asked whether Cruz had the temperament to be president. Over the weekend Trump had called Cruz “a little bit of a maniac.”

Asked about border security, Cruz smiled and said he would build a wall and have “Donald pay for it,” a reference to Trump’s wealth and his signature plan to build a wall along the U.S.-Mexican border.

The friendly exchange allows Cruz to go on presenting himself on the campaign trail as a more credible alternative to Trump for some Republicans, while his team works to build an organization that can turn out more voters than Trump can.

While the Cruz-Trump buddy act continued, there was no love lost between Cruz and U.S. Senator Marco Rubio of Florida. Each sees the other as his main obstacle to climbing higher in the polls.

Cruz and Rubio, both the 44-year-old sons of Cuban fathers and both rising conservative stars in the party, battled over Cruz’s proposal to “carpet-bomb” areas of the Middle East controlled by ISIS.

“ISIS is gaining strength because the perception is that they’re winning, and President Obama fuels that perception,” Cruz said. “That would change when militants across the globe see that when you join ISIS you are giving up your life.”

But Rubio noted that Cruz had voted against defense authorization bills and had supported defense cuts that would make such bombing impractical.

“You can’t carpet bomb ISIS if you don’t have planes or bombs to attack them with,” he said.

With seven weeks to go before the first nominating contest in Iowa, Trump has held or expanded his lead in national polls in the Republican race for the election.

The latest Reuters/Ipsos opinion poll, completed after Trump’s Dec. 7 call for a ban on Muslims, showed him leading the field with support of 33 percent of Republican voters. Cruz was second at 15 percent, followed by retired neurosurgeon Ben Carson at 12 percent, Rubio at 10 percent and Bush at 9 percent.

All five appeared in the main debate along with New Jersey Governor Chris Christie, Ohio Governor John Kasich, U.S. Senator Rand Paul of Kentucky and former corporate executive Carly Fiorina.