Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.) said Friday he wouldn’t rule out a 2020 challenge to President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE while predicting that the president “will have a challenge” from someone in the Republican Party.

Flake, an outspoken critic of Trump who is not running for reelection to the Senate, told CNN's David Axelrod in an interview that while he is not “planning” to run as an independent, he “wouldn’t swear it off.”

“I do think the president will have a challenge from the Republican Party, I think there should be,” Flake said. “I also think that there will be an independent challenge, particularly if the Democrats insist on putting somebody up from the far left of the party.”

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Flake has been one of Trump’s harshest Republican critics since before the president took office, often criticizing Trump over immigration and his rhetoric.

The Arizona Republican announced in October that he would not seek reelection during a dramatic floor speech criticizing Trump and the GOP. Flake accused his party of abandoning "core principles" in favor of "anger and resentment."

“There’s not much place for a Republican like me in a party like this,” Flake told CNN on Friday.

The senator has said publicly multiple times that he would not rule out a presidential run, and expressed concerns that the 2020 race will come down to a choice between Trump and a far-left candidate.

“This polarization we’re seeing on the right is also happening on the left, and there has to be a huge swath of voters in the middle that are looking for something else,” he said.

Trump announced his 2020 reelection bid earlier this week, tapping his former digital adviser Brad Parscale as his campaign manager.