It's official, it's two against two. With mitt romney's selection of paul ryan as his running mate. The race is on. Turns out ryan's favorite movie "the good, the bad and the ugly" may become a theme... See More
It's official, it's two against two. With mitt romney's selection of paul ryan as his running mate. The race is on. Turns out ryan's favorite movie "the good, the bad and the ugly" may become a theme of this year's campaign, as the heat gets turned up. Already in iowa, just miles from president obama who hosted his own rally and the barbs are flying as our senior political correspondent jon karl reports. Reporter: It's game on. The paul ryan pick is already generating more excitement and some of the biggest crowds that mitt romney has ever seen. Go! Go! Go! Reporter: At the iowa state fair today he found excited supports. Congratulations. Hey. Reporter: And a few loud and persistent hecklers. We were right in the middle of it all. -- From iowa -- Reporter: The hecklers have come here from the romney supporters. But ryan didn't let that distract him, talking right over the shouting. I'm guessing the reason -- he only knows left turns. Reporter: As he worked his way through the fair, ryan avoided the funnel cakes and cotton candy and as the fitness buff who swears by the challenging p-90x program. We can play stump the running mate later. Reporter: What do you like about ryan? I think he's conservative. He's right on the money. Reporter: And almost as many who still don't quite know who he is. What do you think of paul ryan? I have no idea. I don't know anything about him. I don't know what I think about him yet. Reporter: What do you think about paul ryan? I don't like him. Reporter: Why not? Because I'm a democrat. Reporter: Ryan is just 42 years old, he said his defining moment came when he was just 16 and found his dad dead of a heart attack, said he was dedicated making his dad proud. A 22-year-old intern, and then by 28, a ngressman. He's done more than just talk about spending. He's put together a plan to slice $6 trillion the next ten years. He actually enjoys looking through the budget for things to cut. I've been reading these things since I was 22 years old. You literally go through it line byly. Reporter: And opposing seniors to buy private insurance yet. What do you say to these maybe weak-kneed republicans? They're a little wobbly. They're going to run the attack ads regardless. This is time for leaders to be leaders. Reporter: Now republicans from mitt romney all the way down are all in when it comes to idea. Hello, iowa! Reporter: President obama was also here, taking a shot at ryan leaving him to block for farm subsidies. Paul ryan might be around in the next few days. He's one of the leaders of congress standing in the way. We've got to put politics aside when it comes to do the right thing for rural america and iowa. Reporter: And there's more to come. Ryan has been the president's nerdy nemesis. Starting with the health care bill three years ago. A bill that's full of gimmicks and smoke and mirrors. Reporter: The following year at a republican conference turned into a at the time tet atet. We'll have a longer debate on the budget numbers there. Reporter: And a hot mike at a fund-raiser caught president obama saying what he really thought about paul ryan. This is the voted for the prescription health care that cost as much as health care but wasn't paid for. Reporter: He may be the number two on the republican ticket but ryan is quickly becoming target number one for democrats. Ryan says he's ready to do battle. And be armed with lots of numbers and a smile. For "nightline," jonathan karl at the I wall state fair.
This transcript has been automatically generated and may not be 100% accurate.