Donald Trump's Lead in Polls Threatened by Ted Cruz's Rising Numbers

The Texas senator recently questioned the mogul's judgement, while Trump called Cruz a "maniac."
3:04 | 12/15/15

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Transcript for Donald Trump's Lead in Polls Threatened by Ted Cruz's Rising Numbers
And we begin with the race for 2016 and what could be a pivotal week. With America soon coming together for the holidays, the candidates want your attention, two giant showdowns ahead. First up, the republicans. They will take to the stage tomorrow night. And once again, Donald Trump will be in the center. And now, we've learned, he's at his highest polling yell. The new poll out tonight showing trump hitting 41% nationally. But there is also something else. In the first key state of Iowa, next month, Ted Cruz is rising in the polls, neck and neck with trump there. The horse race fueling trump's newest attack, comparing Cruz to a maniac, just as trump's doctor is out with a new report on trump's health. ABC's Cecilia Vega is in las Vegas on the eve of the debate. Reporter: After vowing to release his full medical report, tonight, the republican Donald 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." And now, the stage is set for a Donald trump/ted Cruz Vegas showdown. Trump's front-runner standing now facing a very real threat from the Texas senator, who at a private fundraiser recently questioned the reality star's judgment. Trump now on the attack. The way he's dealt with the senate, where he goes in there like a, you know, frankly like a little bit like a maniac. You can't walk into the senate and scream and call people liars and not be able to cajole and get along with people. Reporter: Cruz's response? Tweeting that famous scene in "Flashdance." ? She's a maniac ? ? maniac ? Reporter: But not hitting trump back. Cruz has spent months cozying up to the billionaire. Look, it's not complicated. Number one, I like Donald. But number two, everywhere Donald goes, he brings a hundred television cameras with him. Reporter: Cruz hoping to eventually inherit trump's supporters. But the idea of trump and Cruz leading the pack has some republicans worried, and "Snl" joking it might be time to bring back George W. Bush. The field of republicans out there is so messed up. I figured, it makes you mess me, doesn't it? And Cecilia Vega with us live from has Vegas tonight. And tomorrow night, Cruz standing next to trump? Reporter: Trump is center stage, Cruz, right there next to him because of those new polls. These are two friends. They are men who have been vying for the same conservative primary voters this entire campaign. But David, right here, tomorrow night, though gloves could very welcome off. And you'll be front and center. Cecilia, thank you. And after the republicans battle it out, we hope you join Martha Raddatz and me as we moderate the New Hampshire democratic debate. That's this Saturday, December 19th, 8:00 P.M., 7:00 central right here on ABC.

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

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