Sen. Jeff Flake (R-Ariz.) on Sunday said President Trump could see a Republican or independent challenger in 2020, adding that he's not ruling out a possible White House bid himself.

"I do believe if the president is running for reelection, if he continues on the path that he's on, that that's going to leave a huge swath of voters looking for something else," Flake said on ABC's "This Week."

Flake, who announced he is not running for reelection to the Senate in 2018, has been a vocal critic of Trump and the Republican Party under his leadership.

Flake said Sunday the GOP is largely appealing to "older white men," adding that the party must find a governing philosophy to be successful in the future.

"I do worry, that in the future we'll be faced with a President Trump running for reelection on one side, drilling down hard on a diminishing base, and on the other side you might have, you know, somebody like [Sens.] Bernie Sanders [I-Vt.] or Elizabeth Warren [D-Mass.] on the far left of the Democratic Party," Flake said.

"That leaves a huge swath of voters in the middle," he added.

Flake said Trump is "certainly" inviting a challenge from an independent candidate, and is "probably inviting a Republican challenge as well."

Asked if he's open to running for president in 2020, Flake said, "I don't rule anything out, but it's not in my plans."