Sen. Jeff Flake already has bowed out on a re-election run next year, but the GOP lawmaker isn’t ruling out challenging President Trump in 2020.

The Arizona senator told ABC’s “This Week” that he believes Trump is certain to get an independent challenger if he decides to seek a second term. Flake also added that he wouldn’t be surprised if Trump gets challenged from within the party.

Will that challenger be Flake, who has been fiercely critical of Trump?

“That’s not in my plans” but “I don’t rule anything out,” Flake told the Sunday talk show.

Flake announced earlier this year that he won’t run for re-election in 2018. He faced an uphill battle as polls showed him facing a difficult intra-party challenge from a fellow Republican endorsed by former Trump senior adviser Steve Bannon.

More:Judge partially lifts Trump administration ban on refugees

More:Trump's Christmas message to the troops: 'We're winning'

Flake said Trump was “probably inviting a Republican challenge” as well as an independent candidate challenge.

"People need to see what a traditional conservative looks like ... (and) what it means to believe in the basic tenets of Republicanism,” he said.

Trump and Flake have repeatedly traded jabs. The senator has denounced Trump’s executive order banning Muslims from several countries from entering the U.S., criticized Trump’s penchant personal attacks on opponents, and charged that the current president’s administration has overseen "the regular and casual undermining of our democratic norms and ideals.”

Trump has repeatedly pilloried Flake on Twitter, referring to him as Flake(y) and as a RINO (Republican in name only.)

Most recently, Flake criticized Bannon and Trump’s endorsement of Republican Roy Moore who lost earlier this month to Democrat Doug Jones in a special election for Alabama’s U.S. senate seat. Several women said Moore sought dates and romantic relationships with them when they were teenagers and he was in early 30s. One woman, Leigh Corfman, alleged that Moore initiated sexual contact with her when she was 14.

Flake on Sunday said he believes Moore will be a "lasting" stain on the Republican Party. He added that he's hopeful Bannon's role within the party is being marginalized.

"The last thing we need is to push that ultranationalist, ethno-nationalist, protectionist kind of element of the party,” Flake said. “That's not good for us.”