The mere notion of billionaire investor Mark Cuban running for president in 2020 might be all it takes for President Donald Trump to lash out.

Trump said Cuban was “not smart enough to run for president” in a bizarre and seemingly unprovoked attack on Cuban early Sunday morning.

I know Mark Cuban well. He backed me big-time but I wasn't interested in taking all of his calls.He's not smart enough to run for president! — Donald J. Trump (@realDonaldTrump) February 12, 2017

Cuban initially tweeted he didn’t know what prompted the president’s ire, but added, “Isn’t it better for all of us that he is tweeting rather than trying to govern?”

Hours later, Trump’s motive became clearer.

“Someone close to him told me it was a nypost article saying I was a 2020 threat,” Cuban told The Huffington Post via email Sunday. “That’s all I know.”

Less than an hour before Trump unleashed his criticism of Cuban, the New York Post ran a story on whom the White House was sizing up as potential challenges to Trump’s 2020 re-election bid.

It’s the White House’s “biggest fear” that Cuban, who owns the Dallas Mavericks and is a reality TV star on the investor-themed show “Shark Tank,” will run against Trump and appeal to Republicans and independents with his successful business background, the Post reports.

“He’s not a typical candidate,” one White House source told the paper. “He appeals to a lot of people the same way Trump did.”

Other challengers the White House identified were Sen. Sherrod Brown (D-Ohio), Sen. Chris Murphy (D-Conn.) and Colorado Gov. John Hickenlooper (D).

But Cuban’s wealth ― he’s worth an estimated $3.2 billion ― coupled with his ability to match Trump on claims of being a political outsider, are reportedly making the White House already sweat the next election.

The White House didn’t immediately respond to HuffPost’s requests for comment.

The president was described as being “obsessed” with running for re-election and has already started to lay the groundwork for his next bid.

Cuban said in 2015 he wouldn’t run for president but called it a “fun idea to toss around.”

“If I ran as a Dem, I know I could beat Hillary Clinton. And if it was me vs. Trump, I would crush him. No doubt about it,” he told CNBC at the time.

Cuban later supported Clinton in the general election. When he endorsed Clinton last July, he noted he had hoped Trump would turn out to be a different sort of candidate. Instead, Cuban said Trump had gone “bats**t crazy.”

After Trump insulted Cuban’s intelligence Sunday, Cuban shared an email he had exchanged with Trump last May.

In the letter, Cuban notes, “Everyone is afraid of you. I like to challenge you. And like you said, I may go after that job some day and it could be against you.”