By Steve Holland and Emily Stephenson

LAS VEGAS:

Challenger Jeb Bush lockedhorns with Republican front-runner Donald Trump in a nationalsecurity debate on Tuesday but most rivals avoided tangling withthe billionaire who has emerged as an enduring force in the 2016US presidential campaign.

Their reticence was a nod to Trump's unique ability towithstand attacks and return fire. Last week Trump stunned thefield by proposing to ban Muslims from entering the UnitedStates a move his rivals assailed only to find many Republicanvoters backed the idea and Trump's lead in opinion polls grew.

The Republicans' last debate of the year was a lively affairin a city famed for heavyweight boxing matches. Although Bushassailed 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 chaoscandidate and he would be a chaos president" Bush 62 said.

Trump 69 shot back: "Oh yeah and you're a tough guyBush" noting Bush's falling standing in the polls and hisresulting move down toward the end of the debate stage.

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

Voicing appreciation for the response he had received on thecampaign trail Trump seemingly ended any further speculationthat he might bolt from the Republican Party and run as an

independent candidate which would almost certainly mean aDemocrat would win the White House in the Nov. 8 election.

"I am totally committed to the Republican Party. I feel veryhonored to be the front-runner" Trump said.

Cruz-Trump buddy act

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

With Cruz's criticism on an audio recording there was anexpectation that Trump and Cruz would finally set aside theirtruce especially after Cruz had seized the lead in a major pollthis 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 theformer reality TV star Cruz demurred.

"What I said in private is exactly what I'll say here whichis that the judgment that every voter is making of everyone ofus up here is who has the experience who has the vision whohas 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 whenasked whether Cruz had the temperament to be president. Over theweekend Trump had called Cruz "a little bit of a maniac."

Asked about border security Cruz smiled and said he wouldbuild a wall and have "Donald pay for it" a reference toTrump's wealth and his signature plan to build a wall along theU.S.-Mexican border.

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

Cruz-Rubio clash

While the Cruz-Trump buddy act continued there was no lovelost between Cruz and U.S. Senator Marco Rubio of Florida. Eachsees the other as his main obstacle to climbing higher in thepolls.

Cruz and Rubio both the 44-year-old sons of Cuban fathersand both rising conservative stars in the party battled overCruz's proposal to "carpet-bomb" areas of the Middle Eastcontrolled by Islamic State militants also known as ISIS.

"ISIS is gaining strength because the perception is thatthey're winning and President Obama fuels that perception"Cruz said. "That would change when militants across the globesee that when you join ISIS you are giving up your life."

But Rubio noted that Cruz had voted against defenseauthorization bills and had supported defense cuts that wouldmake such bombing impractical.

"You can't carpet bomb ISIS if you don't have planes orbombs to attack them with" he said.

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

The latest Reuters/Ipsos opinion poll completed afterTrump's Dec. 7 call for a ban on Muslims showed him leading thefield with support of 33 percent of Republican voters. Cruz wassecond at 15 percent followed by retired neurosurgeon BenCarson at 12 percent Rubio at 10 percent and Bush at 9 percent.

All five appeared in the main debate along with New JerseyGovernor Chris Christie Ohio Governor John Kasich U.S. SenatorRand Paul of Kentucky and former corporate executive Carly

Fiorina.

Reuters