Ben Carson, From Surgeon to GOP Candidate

What the Republican presidential hopeful thinks about Donald Trump as he aims to be the top contender.
5:53 | 09/16/15

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Transcript for Ben Carson, From Surgeon to GOP Candidate
Hurry, offer ends September 19th. Get out of the past. Get fios. Diabes? Wh if one t-up cou prent heardisease? On Wiful thing, right Bu you N take toelp prevt another Seris disease Pnmococcalneumonia Ifou are 5or older evnar 13 vaccine cahelp it's called tomorrow night. Reporter: On the eve of the hotly anticipated second gop debate, entrenched front-runner Donald Trump ready for the attacks against him. How is the debate prep going? Very good. Reporter: There is one guy giving him a run for his money, from out of the shadows a new challenger emerging. Nearly neck and neck with trump in the latest polls. Debates are coming up, are you nervous? Preparing? What are you doing? It's not brain surgery. Reporter: Retired brain surgeon would know. Of course, you know, I prepare every day. And I'm learning more things every day. Reporter: A "New York times"/cbs news poll released this morning has Ben Carson fully established as number ber two candidate within striking distance from trump. Do you worry you may trip or something? No. Reporter: "Nightline" was with Dr. Carson on the trail just days ago in texas. That's what I call a Texas welcome. When you come out to a rally like this, what's going through your head? I haven't really even thought about how I'm going to start or what I'm going to talk about. That will all come to me when I get up there. You just basically go extemporaneous? Yes. Reporter: Surprising admission from a man from planning surgeries but one who speaks to his growing appeal among republican voters as a political outsider. Do you have lines that you go to that you know are applause lines? No. Not really? No, I don't, really. I've watched politicians do that. That doesn't appeal to me. Reporter: It was however a surprising zinger that helped him being a standout on fox. I'm the only one who separated siamese twins. Reporter: And the moment some believe his campaign really took off. The only one to operate on babies while they were still in a mother's womb, the only one to take out half of a brain although you think if you go to Washington that someone had beat me to it. This is going on the interior frontal. Reporter: ABC news was in the operating room in 1999 filming for a special "Hopkins 24/7" when Dr. Carson successfully removed half the brain of a 4-year-old girl who suffered debilitating seizures. Making life and death decisions and profound decisions that affect the quality of people's lives is an awesome responsibility. Reporter: A statement with a certain kind of residents for a man looking to become president. Everything went very smoothly. No problems whatsoever. So the operation is all done. All your prayers worked. What was that like? I never, through all of those years, 15,000 operations, lost my awe of the brain. Every time I would take off that cranial bone and open the door and there would be the brain. I would say, wow, that's the thing that makes this person who they are. And I'm also fascinated still by the fact that, in a child, we can take half of that out and they can still function cc1 Test message Text1 underline Test In many ways he's the opposite of Donald Trump. Dr. Ben Carson soft-spoken, not a big fan of verbal trench warfare, but Carson who has emerged in the polls as trump's main competitor will be standing right there next to the the republicans meet for their second debate. NBC takes us on the trail and inside the O.R. With the doctor. Hottest thing out there. This hat. Reporter: It's a campaign at times pulling out more like a reality show than a race for president. I'm here for a certain reason. You know, teenage rage. One day when I was 14 years old, another youngster with doing something that I didn't like -- Reporter: Here in a 1990 video Carson tells a group of young people how a moment of violence in his childhood cemented his faith. I happened to have a large camping knife on me. Nd and I grabbed it and tried to stab him in the abdomen with it. Figured I would put him out of his misery. Well, fortunately under his clothing he had on a very large metal rotc belt buckle. That knife blade hit that belt buckle and broke, and he took off. You know, I was even more horrified than he was because I recognized at that moment that I was completely and totally out of control. And I started praying and asking god to take that temper away from me. Reporter: So it's easy to see how today Carson is unfazed when his faith is questioned by Donald Trump. He characterized you maybe as phony Christian and a phony conservative. What do you say to that? I would say the record speaks for itself. I don't think people have to go very far to see what my belief system has been. Reporter: Carson hoping to turn around the country like he turned around his life, a case he will try to make on that debate stage. For "Nightline" in Los Angeles.

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

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