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President Trump, in a Monday morning interview with "Fox & Friends," said the U.S. is "never going to have a fair" presidential election if universal mail-in voting is widely allowed.

Trump reacted to media pundits who discussed the possibility that he may refuse to leave office if he didn't accept the result of the presidential election. Trump warned the election may be tainted by fraud stemming from mail-in voting.

"I have to tell you, that if you go with this universal mail-in ... tens of millions of ballots being sent to everybody and their dogs, dogs are getting them okay, people that have been dead for 25 years are getting them, you have to see what's happening. Then you're never going to have a fair election," Trump said. He later added that if universal mail-in voting were allowed to go forward, it would be "the greatest scam of all time."

Trump did, however, say that he supported absentee ballots that required a voter to request a ballot and additional in-person voting options.

"Absentee ballots like in Florida, those are great things. you send for it. you ask for it. They send it to you, you send it back with your vote. Those are great," Trump said.

"We voted during many crises," Trump said of past American elections, while also decrying some of the recent mail-in voting fiascos.

Fewer than 10 states are conducting universal mail-in voting, in which ballots are sent to every registered voter. Other states are taking other measures, including sending absentee ballot applications to every voter.

In another exchange, the president said Democrats will struggle to unite at their convention this week, as many of the more progressive members of the party balk at a ticket including former Vice President Joe Biden and Sen. Kamala Harris, D-Calif.

"These are the same people I beat last time and they're not gonna unite because they can't unite, they don't know how to unite," Trump said. "Also, Bernie [Sanders] is very unhappy, but actually, Biden went further left than Bernie wanted him to and Bernie's still unhappy."

Trump added: "You'll never make this guy happy ... I call him crazy Bernie for a reason."

The Vermont senator, despite Trump's comments, has fully backed Biden's campaign. Still, some of his delegates and vocal supporters have criticized Biden and Harris as his vice-presidential pick. A petition circulated late last month by Sanders delegates to vote against the DNC platform because it did not include "Medicare-for-all." The DNC rules committee is set to report its vote Monday.

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Trump's interview comes ahead of the first day of the DNC and as the 2020 presidential campaign nears the home stretch.

Trump and his campaign are introducing their own counter-programming to the DNC with Trump campaigning in the swing states of Minnesota and Wisconsin on Monday before speaking in Arizona on Tuesday. His campaign, meanwhile, will hold a post-convention show at 11 p.m. each night of the DNC, starting Monday.

"We won it last time, I think we're going to win it this time," Trump said of Wisconsin.

"I have no choice because we have a fake media in this country, so I have to work, I don't have the time not to," Trump said of his swing state visits this week, while also criticizing Biden for running his campaign largely from "his basement."

Trump addressed the criticism of his response to the coronavirus crisis and more.

"They should be able to get this equipment themselves, but when they can't, we've got equipment like nobody's ever seen before," Trump said. "They say it's the biggest mobilization since World War II."

He discussed Democrats' efforts to pass the coronavirus stimulus bill, which he said was really about their desire for "$1 trillion to bail out their badly run states, like New York State, which is horribly run;" and trade with China, which he said in past decades had been a "one-way street" but noted that the country just "ordered the largest order of corn in history."

The morning show hosts also asked Trump how he was preparing for the upcoming presidential debates. Trump said he wasn't spending much time worrying about them.

"By working very hard ... not on debates but on running the country," Trump said. "Honestly, what I'm doing is, I'm doing my job. My job is ... that's the best debate prep ... I guess I'll do some preparation but I didn't so much last time because I understand what's happening. I mean, law and order, we need law and order. You don't have to prep for that."

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On what Biden brings to a debate, Trump said "he was okay" in some debates against Sanders but was "so bad" in other debates.

Trump also addressed the death of his brother Robert, who he called "one of the most loyal people" and "a fantastic guy."

Fox News' Morgan Phillips contributed to this report.