Former Senate Majority Leader Harry Reid, D-Nev., issued a warning to Democrats ahead of the 2020 election: Do not underestimate President Trump.

In an interview with CNN's "Axe Files," Reid told host David Axelrod that he disagrees with anyone who believes Trump will be easy to beat, saying they "have another thing coming."

“[Trump] is a man who has a stalwart 40 percent of people out there who will vote for him no matter what he does. As Donald Trump said, 'I could shoot somebody in Times Square, they would still support me.' That’s sad, but probably true. So No. 1, I repeat: He is not going to be beaten easily. He’s going to take a campaign of wisdom and patience, but he is beatable for sure," said Reid, who retired in 2016.

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Reid, 79, announced in May 2018 that he had been diagnosed with pancreatic cancer. This past February, he said his cancer had gone into remission after treatment.

In the interview, Reid said he used to believe Trump was not very smart, but his view has changed.

“I don't think he's intellectually a powerhouse, but I think he's basically a very, very smart man. No matter what the subject, any argument he involves himself in, it's on his terms. You're always arguing against him," said Reid, adding that Democratic candidates cannot underestimate Trump going forward.

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Reid said he is "disappointed" in Senate Republicans for refusing to stand up to Trump, arguing the evidence is "very clear" that Trump sought to withhold military aid to pressure Ukraine to help investigate Joe Biden and his son.

"We have these Republicans that are afraid to speak out against the things he does that are absolutely wrong and they know they are wrong," he said, crediting Republican Sens. Mitt Romney and Ben Sasse for criticizing Trump.