Transcript for Bush and Rubio Bond Giving Way to a Fierce Rivalry
And on the eve of the big republican debate, it's not just trump and Carson battling it out. Jeb bush and Marco Rubio. Bush, one of Rubio's mentors. They are now being forced to fight it out themselves. In fact, during a presentation in a closed-door meeting with Jeb bush supporters, the bush team revealing what they're now calling Marco Rubio. ABC's chief white house correspondent Jonathan Karl. Reporter: Jeb bush used to talk about Marco Rubio like a proud father. I'm so proud of his enthusiasm. I'm so proud of his eloquence. And I'm so proud that he will be part of a next generation of leaders that will restore America. Reporter: Now bush has come to see rube yoel as target number one. His main competition to becoming the alternative to outsiders trump and Carson. He's hammered him for missing senate votes. If congressmen and women are elected to serve, they actually ought to show up and vote. Reporter: And that presentation to big donors flatly declared Marco as a G.O.P. Obama, the ultimate fighting words in a republican primary. For his part, Rubio has suggested his former mentor is part of the past. I'm going to continue to focus on what I think our country needs to do and why it is I think we need to elevate a new generation of leadership. Reporter: Fighting with his former friend and protege is perhaps another reason Jeb bush sounds so disgusted with the race. Ill got a lot of really cool things they could do other than sit around being miserable, listening to people demonize me and me feeling come peopled to demonize them. That is a joke. Elect trump if you want that. And Jon Karl with us live here in New York. Always great to have you here on the set with us. We want to show everyone at home another poll out tonight. And this one is on the democratic side. It shows Hillary Clinton with a commanding lead now in Iowa, 61 points ahead of Bernie sanders. Does this mean the only state where she's not leading is new Hampshire? Reporter: That's right. We haven't seen a poll in new Hampshire since the benghazi hearings. But sanders had been looking like he could actually beat Hillary Clinton in both Iowa and New Hampshire the first two states. Now, once again, she's looking like the prohibitive front-runner in Iowa and nationally. But David, we saw this in 2008 and the beginning of this campaign. What happens when she becomes the all-out front-runner. Jon Karl, thank you.
This transcript has been automatically generated and may not be 100% accurate.