Countdown to Iowa Caucus: Will Donald Trump Win?

Presidential hopeful Ted Cruz takes the lead over the real estate mogul in a newly released poll.
5:19 | 12/13/15

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Transcript for Countdown to Iowa Caucus: Will Donald Trump Win?
Paula is off. It's great to have Cecilia Vega here on a morning where there's a big political story brewing. That's right. A look at a brand-new Des Moines register poll. Ted Cruz cruising past Donald Trump to take the lead in the key state of Iowa. The words of the Des Moines register, Cruz is quote crushing it. Now leading trump by 10%. Since the newspaper's last poll in October, Cruz up a stunning 21 points. Meanwhile, Ben Carson down by 15%, dropping to third place. What does this mean for the race and what is trump saying about all of this. George stephanopoulos Z standing by. But let's start with Mary Bruce at the white house this morning. Hi, Cecilia, good morning. After dominating the Iowa polls for five months, Donald Trump is now out of first place. This poll is the second one this week to show Ted Cruz on top in the key state. This morning, Donald Trump is in an unfamiliar place, second place. Ted Cruz plough past trump in the did new Des Moines register poll. It's a jaw-dropping rise for the republican. 31 .leap. Cruz tweeting overnight, thank you. Meanwhile, trump was quick blast to new Numbers. Don't trust Des Moines register poll, it's biased. Trump has been clear he's not a fan of the paper. The Des Moines register is the worst. They are the worst. The worse. No, their very dishonest. Reporter: Despite falling to second, trump maintains strong strength in the poll. Trump has been publicly taking aim at Cruz this week. A difference of temperaments and I would give myself the advantage in temperament and you know, putting people together. That's a question of strength but it's also a question of judgment. Reporter: Meanwhile, Hillary Clinton getting a big boost from the president. President Fitzgerald grant. Campaigning for the democratic front-runner in Iowa. She's by far the most qualified and most experienced. Now this big shakeup in the gop race comes two days before the candidates face off in their fifth debate on Tuesday. All eyes will be on trump and Cruz. Everybody wondering if they're going to play nice. Let's bring in ABC news chief anchor George stephanopoulos who's going to be hosting "This week" later this morn. If you're Donald Trump going into this debate, do you attack Ted Cruz? They already started doing it Friday night in Iowa. Started to question whether Ted Cruz is a real evangelical. Also started attacking him for not being for ethanol sub subsidies in the state of Iowa. This poll as Mary was talking about is huge. For Ted Cruz. If you combine, those who his first and second place, he has strong favorability ratings now. There's another pretty eye-popping development in this poll, the rather stunning decline of Dr. Ben Carson, what's going on here? Well, a huge drop. Almost 15-point drop since October. One thing you're seeing with national security, top of the news, right now in the news in the wake of San Bernardino and in the wake of Paris. Who do you trust as commander in chief, he said that Carson is falling on that measure. Let me ask you about the current commander in chief, president Obama came out yesterday to celebrate this landmark agreement among 195 nations to tackle climate change. So, two-part question, how big of a deal is for this president's legacy and for the planet? It's something that he's been pushing very hard through his presidency. Whether these 195 nations actually follow through on these commitments on carbon emissions. Looking here at home, you see all of the republican candidates against the deal, they're vowing to undo the steps that president Obama has taken to implement the reductions. Debates like that all over the world. Will it stick? That's the question. It depends on the political commitment. George, thank you very much. Appreciate it. I want to remind everybody, George has a big show this morning, including an exclusive one-on-one with Ben Carson. Looking forward to that. Also looking forward to the next democratic debate this coming Saturday debate. December 19th. We do now move on to that dangerous weather slamming Texas

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

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