GOP Presidential Candidates Prepare for 5th Republican Debate

Texas Sen. Ted Cruz is now posing a real threat to Donald Trump's front-runner status.
3:00 | 12/15/15

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Transcript for GOP Presidential Candidates Prepare for 5th Republican Debate
tomorrow. No spoilers here. I'm on debate duty. Look at the strip right there. Hottest ticket in town. Donald Trump right now, the strong front-runner. They are preparing for a prize fight. We'll tick a look inside the venetian where the debate will take place, nine candidates up on that stage, Donald Trump center stage once again. The gop front-runner growing even stronger in the polls topping our new ABC news/"washington post" poll with 38% but that trump rally last night got ugly and ABC's Cecilia Vega starts is off in Las Vegas. Good morning, Cecilia. Reporter: Robin, good morning to you. Because of those polls Ted Cruz for the very first time will be standing right next to Donald Trump on that stage here in just a few hour, but as you mentioned about that rally last night very disturbing allegations about things that were said as protesters were being taken out of that room. Take a listen. Overnight a Donald Trump rally in Las Vegas turning ugly. Protesters repeatedly interrupting his speech as a man is being removed. It's unclear who but someone yells "Light him on fire." Light the On fire. Bye-bye. Reporter: Overnight Donald Trump lobbing a preemptive strike across the bow at the man coming dangerously close to his lead. Trump, 41%, Cruz, 14%. And everyone else much lower, I mean, somewhere. Reporter: Texas senator Ted Cruz posing a very real threat to the reality TV star's front-runner status and before a huge crowd in Las Vegas, trump ready for debate night with unshakable confidence. I'll be honest, I think tomorrow night, I think it's going to be big. Oh. Ooh. And they're all coming after me. So far everyone that's attacked me has gone right down the tube so that's good. Reporter: For now Cruz still playing nice. I don't think the American people are interested in a bunch of candidates bickering like schoolchildren. Reporter: But the stage now set for a showdown on the strip just hours away. Florida senator Marco Rubio spending his debate eve rallying supporters in Las Vegas too going after president Obama rather than the eight rivals who will share the stage with him. So last Sunday the president spoke to the nation in an effort to calm us and make us feel more comfortable. I swear to you I wish he hasn't spoken at all. Reporter: 69-year-old trump says he's ready and healthy enough for the fight. The billionaire offering a clean bill of health as proof. Well, sort of. After vowing to release his full medical report, trump instead offered up just this short note from his doctor saying trump's physical strength and stamina are extraordinary. And in true trump fashion, the doctor declaring that if elected, Mr. Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency. The healthiest individual and that letter declaring trump astonishingly excellent but one glitch, trump said that the doctor who wrote that letter was named Jacob Boren Steen. Jacob Boren Steen passed away years ago. His son wrote that letter and trump fixing that later post but a bill political oops out here.

This transcript has been automatically generated and may not be 100% accurate.

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