WATCH: Arizona Sen. Jeff Flake, a Trump critic, will not seek re-election

Sen. Jeff Flake, R-Ariz., announced in an impassioned speech on the Senate floor that he will not seek re-election in 2018, decrying the coarse tone of politics and "flagrant disregard for truth or decency" ushered in by the election of President Trump.

"We must never regard as 'normal' the regular and casual undermining of our democratic norms and ideals," Flake said.

"We must never meekly accept the daily sundering of our country — the personal attacks, the threats against principles, freedoms and institutions; the flagrant disregard for truth or decency; the reckless provocations, most often for the pettiest and most personal reasons, reasons having nothing whatsoever to do with the fortunes of the people that we have all been elected to serve.

"None of these appalling features of our current politics should ever be regarded as normal," Flake continued.

Flake has been a frequent critic of Trump since the 2016 campaign and, as a target of right-wing frustration himself, has been seen as vulnerable to a GOP primary challenge in 2018. He was a top target of former Trump chief strategist Steve Bannon for defeat in the GOP primary, and early polls showed Flake trailing former Arizona state Sen. Kelli Ward.

Flake even penned a book earlier this year that was a damning rebuke of Trump's populist and nationalist agenda, accusing the GOP of making a "Faustian bargain" when the party overlooked Trump's abandonment of longtime conservative principles like free trade and overlooked many of Trump's controversial statements and stances.

But Flake's speech on the Senate floor was an unprecedented indictment of a sitting president by a member of his own party. Flake said that he was speaking because "there are times when we must risk our careers in favor of our principles" but that his comments nonetheless came "with no small measure of regret."

"Regret, because of the state of our disunion, regret because of the disrepair and destructiveness of our politics, regret because of the indecency of our discourse, regret because of the coarseness of our leadership, regret for the compromise of our moral authority, and by our — all of our — complicity in this alarming and dangerous state of affairs. It is time for our complicity and our accommodation of the unacceptable to end," Flake implored his fellow Republicans.

"In this century, a new phrase has entered the language to describe the accommodation of a new and undesirable order — that phrase being 'the new normal.' But we must never adjust to the present coarseness of our national dialogue — with the tone set at the top," Flake continued, with a clear shot at Trump.

Flake first informed the Arizona Republic of his decision, telling the newspaper "there may not be a place for a Republican like me in the current Republican climate or the current Republican Party."

Flake's announcement came on the same day that Sen. Bob Corker, R-Tenn., who is also retiring, also unloaded on Trump amid their ongoing Twitter feud. Corker said that "the constant nontruth-telling, just the name-calling, I think the debasement of our nation" is what the president will be remembered by.

Flake's seat is a target for Democrats hoping to pick up a seat in a year that looks very good for Republicans in the Senate, and Republicans believed that if he lost the primary, the seat would be much harder to hold. Now, his decision to retire throws the prospects for his seat even more up in the air.