Joe Biden calls Trump 'an existential threat to America'

In one speech in Iowa, Biden mentioned the president 76 times; Meanwhile, President Trump attacked the former vice president's stamina and mental fitness.
2:58 | 06/12/19

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Transcript for Joe Biden calls Trump 'an existential threat to America'
Now to the race for 2020 tonight, and to the faceoff this evening in Iowa. President trump and Joe Biden both campaigning there. Air force one there on the tarmac near Omaha, landing just a short time ago with the president onboard. Joe Biden already there, both already tonight taking aim at each other. The gloves are coming off, and this evening, the new and eye-opening numbers. If this does end up being trump versus Biden. Mary Bruce from Iowa tonight. Reporter: President trump heading to Iowa today where Joe Biden was waiting for him. The former vice president making his most direct attacks yet. The president is literally an existential threat to America. Reporter: Speaking to voters in Iowa, Biden said the president undermines American values. But this is a guy who does everything to separate and frighten people. It's about fear and loathing. No president has done something like that, for god's sake. I mean, it's bizarre. And it's damaging. Reporter: Biden is eager to look past his democratic rivals. In just one speech today, he references the president 76 times. Trump is paying close attention. He was some place in Iowa today and he said my name so many times, people couldn't stand it anymore. No, don't keep saying it. Sleepy guy. Reporter: The president taking a page out of his 2016 playbook, attacking the former vvpt's stamina and mental He looks different than he used to. He acts different than he used to. He's even slower than he used to be. I'd rather run against, I think Biden than anybody. I think he's the weakest mentally. And I like running against people that are weak mentally. Reporter: Mr. Vice president, why do you think the president says he would rather run against you thaun anyone else? That's -- I don't know. I have no idea. I'm looking forward to it. Reporter: Biden is leading in the early polls, but with 23 democratic candidates to choose from, some Democrats we spoke with today aren't sold yet. He's a person I really respect, I loved him as vice president, but I think he has too much baggage he's accumulated over the years. Reporter: Democrats say it's time for new blood in the party. What do you make of that? We need stability. Reporter: Stability? Stability. We need somebody that knows the ropes. We have a long way to go. Mary Bruce already in mt. Pleasant, Iowa, for us. And Mary, you're aware of this new poll out tonight, a national poll showing the potential match-up, if this is former vice president Biden versus trump. Biden leading the president by a pretty wide margin, 53% to 40%. But that same poll, Mary, shows Biden leading the pack of Democrats, but it appears one candidate, it would seem, is on the move as of late. Reporter: Well, David, Biden is brushing off the polls. He says this is just the beginning of a very long marathon. While the president says these new numbers are fake news. Along the Democrats, Elizabeth Warren is on the move. She's now in third place nationwide, just four points behind Bernie Sanders. David? Mary Bruce, thank you. Next tonight, to the

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

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