Transcript for GOP Debate: Candidates Clash Over National Security
Donald Trump rolled in to Vegas tonight with the most to lose. The clear republican front-runner knew he had a target on his back heading into this, the final debate of the year. But he was not the only one who took fire. Here's ABC's David Wright. ? Reporter: The Las Vegas strip hosts plenty of prize fights but tonight at the venetian this one was big even by Vegas standards. And bloody, even if most of the prize fighters wore dark suits, red tie, and a flag pin. Am I talking or are you talking, Jeb? I'm talking. You're not talking. You interrupted me. Reporter: Tonight Jeb bush showed up for the fight, landing some punches on front-runner Donald Trump. Donald, you're not going to be able to insult your way to the presidency. That's not going to happen. I do have the strength. Leadership -- leadership is N not -- leadership is not about attacking people and disparaging people. Leadership is about creating a serious strategy to deal with the threat of our time. With Jeb's attitude we will never be great again. That, I can tell you. We will never be great again. Reporter: This was the last gop debate of 2015. The first run, since the terror attacks in Paris and San Bernardino, also the first gop debate since Donald Trump vowed this -- For a total and complete shutdown of muslims entering the United States until our country's representatives can figure out what the hell is going on. Reporter: Tonight as expected -- Border security is national security. And we will not be admitting jihadists as refugees. Reporter: Terrorism and trump were both center stage. Donald, you know, is great at the one liners. But he's a chaos candidate. He would be a chaos president. Jeb doesn't really believe I'm unhinged. He said that very simply because he has failed in this campaign. It's been a total disaster. Reporter: Bush wasn't the only one gunning for him. Rand Paul took some shots, too. The question is, how do we peep America safe from terrorism? Trump says we ought to close that internet thing. The question really is, what does he mean by that? Like they do in North Korea? Reporter: Trump's reaction to all of these attacks spoke volumes. At one point trump actually blamed CNN for egging bush on. I think it's very sad that CNN leads. Jeb bush, governor bush, down a road by starting off virtually all of the questions, Mr. Trump this, I think it's very sad. If you think this is tough and you're not being treated fairly, imagine what it's going to be like dealing with Putin or dealing with president xi or the islamic terrorism that exists. This is a tough business run for president. You're a tough guy, Jeb. We need to have a leader. You're never going to be president of the United States by insooting your way to the presidency. Let's see. I'm at 42% and you're at 3%. So far I'm doing better. I was surprised at his aggressive attitude toward Donald Trump. I thought it important he be more awe gresive and assertive but I thought he would have aimed at others. Reporter: Dr. Ben Carson treated like a front-runner last time around, this time he struggled to get a word? Some people say you're weak because you're not loud and you're not rude, but the fact of the matter is, look and see what I've done, and that speaks volumes about strength. I think that probably the thing that came out of this was a continual slide of Ben Carson who led at one point in time and afterward the tragic events in France when people started focusing more on these issues related to security he started falling. I think the course of this debate that fall continues. And my expectation is Ben Carson will be in seventh or eighth place by the next couple of weeks. Reporter: The ratings of these debates have rivalled prime time TV shows and sporting events. I think we've run out of red ties in the Las Vegas area. Reporter: Tonight in queens the qed cafe had it on big screens. Think about what you did, Marco. Reporter: Comedians and commentators weighing in in realtime on the action. Jeb start looking like when George won the presidency and he realized he was never going to be as good? It's almost like watching WWE at this point. I think that's the appeal of Donald Trump, is that it's like watching macho man randy savage with, you know, even worse hair. Reporter: Throughout the campaign trump's fiery rhetoric hasn't exactly hurt him. Quite the contrary. But senator Ted Cruz has been gaining ground, several recent polls show him slightly ahead of trump in Iowa. Until now the two have mostly abided by a mutual nonaggression pact. You better not attack. Reporter: Even tonight Cruz studiously avoided a full frontal attack on trump's proposal to ban muslims from the U.S. Everyone understands why Donald has suggested what he has. Reporter: Though he did distance himself from it. There are millions of peaceful muslims across the world and countries like India. It's not a war on a faith. It's a war on a political and theocratic ideology that seeks to murder us. Reporter: The turmoil up top has left the rest of the field to jockey into position themselves as the reasonable alternative. Tonight senator Marco Rubio tangled with Ted Cruz over immigration. Does Ted Cruz rule out ever legalizing people that are in this country illegally now? Senator Cruz? Do you rule it out? I have never supported legalization and I do not intend to support legalization. Reporter: Their conflict sounding at times like a vice presidential debate, auditioning for the second spot on trump's ticket. We will build a wall that works, and I'll get Donald Trump to pay for it. I'll build it. Reporter: Governor Chris Christie painted himself as a seasoned executive but he may not have helped himself with women voters with an analysis of today's terror scare in L.A. That sounded straight out of "Father knows best." Think about the mothers who will take those children tomorrow morning to the bus stop wondering whether their children will arrive back on that bus safe and sound. Think about the fathers of los Angeles who tomorrow will head off to work and wonder about the safety of their wives and their children. Reporter: Does any of this have the potential to change the race? I think Donald Trump comes away as still the leader. And I think Ted Cruz and Marco Rubio are the people closest at his heels. I think Jeb bush tried in his way to be aggressive tonight but I think it's probably too little too late. Reporter: Voters at the qed screening party tonight aren't sold. I really couldn't pick a winner. Lost me halfway through. Reporter: As of tonight, there are less than 50 days to go until the Iowa caucuses. Soon enough that voters there will have their say. I'm David Wright for "Nightline" in New York.
This transcript has been automatically generated and may not be 100% accurate.