Trump attacks Mark Cuban on Twitter as report says Cuban is his 'biggest fear' in 2020

US President Donald Trump took to Twitter on Sunday morning to attack billionaire Dallas Mavericks owner Mark Cuban.

"I know Mark Cuban well. He backed me big-time but I wasn't interested in taking all of his calls," Trump tweeted, adding that Cuban is "not smart enough to run for president."

The attack came amid a report from the New York Post, which cited a person close to the White House as saying that the businessman is Trump's "biggest fear" in the 2020 presidential election.

The report also singled out Democratic Sens. Sherrod Brown and Chris Murphy, as well as Colorado Gov. John Hickenlooper, as possible opponents to Trump in 2020, but stressed that Cuban posed the biggest perceived threat.

"He's not a typical candidate," a source told the Post. "He appeals to a lot of people the same way Trump did."

Cuban regularly criticized Trump's candidacy, becoming a key surrogate for Democratic nominee Hillary Clinton. After an initial grace period with Trump, he has resumed his criticism with intensity as of late.

"I've been crushing POTUS," he wrote on Twitter at the end of January. "He has earned it."