Arizona Sen. Jeff Flake isn’t ruling out a 2020 presidential run to challenge President Trump, either as a Republican or as an independent candidate.

Flake said on ABC’s “This Week” he’s not planning on making a run at the moment, but “I don’t rule anything out.”

“That's not in my plans. I do wonder and I do worry that in the future we'll be faced with President Trump running for re-election on the one side, drilling down hard on a diminishing base. And on the other side, you might have somebody like Bernie Sanders or Elizabeth Warren on the far left of the Democratic Party,” he said.

“That leaves a huge swath of voters in the middle that may be looking for something else.”

He said later, "I don't rule anything out, but it's not in my plans."

When pressed if he would run as a Republican or an independent, Flake demurred.

“I haven't thought that deeply about it. Like I said. But I do — I do believe if the president is running for re-election, if he continues on the path that he's on, that's going to leave a huge swath of voters looking for someone else,” he said.

Flake is retiring from the Senate next year at the end of his term. He said in a speech on the Senate floor that he chose not to run for re-election because he believed he’d be required to attach himself to Trump and do things he’s not willing to do in order to win the Republican primary.

Flake is a staunch critic of Trump’s behavior while in office but has mostly toed the Republican Party line with his votes in the Senate.

He said the Republican Party needs to realize what becoming the party of Trump really means for its electoral future.

“You look at the audiences cheering for Republicans sometimes, you look out there and say, those are the spasms of a dying party,” he said. “When you look at the lack of diversity sometimes, and it depends on where you are, obviously.”

“But, by and large, we're appealing to older white men. And there are just a limited number of them. And, anger and resentment are not a governing philosophy. So, you have to actually govern and do something.”