Arizona Sen. Jeff Flake once said the "presidency has been debased" by Donald Trump, saying the commander-in-chief has a “bottomless appetite for destruction and division.”

Tennessee Sen. Bob Corker has referred to the White House as an "adult day care center," and has doubted whether Trump has “the stability nor some of the competence” to serve as president.

Both Flake and Corker established themselves as two of Trump’s most prominent Republican critics on Capitol Hill during the first two years of his presidency.

Later this week, both Republicans will be gone from Washington.

Their retirements from the Senate raises the question of who -- if anyone -- will take their place as willing to publicly criticize a president who remains popular with nearly 9 in 10 Republican voters.

Flake and Corker engaged in a war of words with the president on myriad issues over the past 18 months, generating headlines and fiery tweets from a president who generally insists on getting the last word. Those battles put them on the outs with many in their own party, and they paid a price. Both decided to retire rather than take on a difficult re-election campaign.

Flake was far and away Trump's most consistent critic among Senate Republicans. Corker weighed in less often, but his description of the White House as an "adult day care center" rankled the president, who dubbed him "Liddle' Bob Corker." The feud continued as Corker headed for the exits, with Trump asserting that Corker's promise to serve only two terms was not the real reason he retired. Rather, Corker "wanted to run but poll numbers TANKED when I wouldn't endorse him," Trump tweeted.

Corker replied: "Yes, just like Mexico is paying for the wall... #AlertTheDaycareStaff."

For now, don't expect any Republican senator to take their place as chief agitator when the new Congress convenes Thursday.

But two other Republicans, Sen.-elect Mitt Romney of Utah and Sen. Ben Sasse of Nebraska, have both had their public run-ins with the president.

Flake is among those mentioned as possible Republican challenger to Trump in a 2020 presidential primary. He hasn't shu the door on a possible White House run.

JEFF FLAKE REPORTEDLY CONSIDERING 2020 TRUMP CHALLENGE: 'SOMEBODY NEEDS TO RUN ON THE REPUBLICAN SIDE'

The Arizona lawmaker said in November that he hasn’t “ruled it out,” nor has he “ruled it in.”

“Just, somebody needs to run on the Republican side,” Flake said, adding that he thought Ohio Gov. John Kasich and Sasse would also be good Republican alternatives to challenge Trump in 2020.

“I hope somebody does [run], just to remind Republicans what it means to be conservative and what it means to be decent. We’ve got to bring that back,” Flake said. “You can whip up the base for a cycle or two but it wears thin. Anger and resentment are not a governing philosophy.”

Fox News' Elizabeth Zwirz and The Associated Press contributed to this report.