A pair of Republican senators sounded an alarm Tuesday about President Trump's fitness for office and warned that his actions were degrading and dangerous to the country — an extraordinary breach that threatens his legislative agenda and further escalates the civil war tearing apart the Republican Party.

Delivering an emotional speech on the Senate floor announcing that he will not seek reelection next year, Sen. Jeff Flake (R-Ariz.) said Trump's behavior is "dangerous to our democracy" and summoned fellow Republicans to denounce the president's conduct.

"It is time for our complicity and our accommodation of the unacceptable to end," Flake said. He added, "Politics can make us silent when we should speak, and silence can equal complicity."

The charged remarks from Flake — a totem of traditional conservatism who has repeatedly spoken out about his isolation in Trump's GOP — came hours after Sen. Bob Corker (R-Tenn.) questioned the president's stability and competence, reigniting a deeply personal feud with the president.

Corker, the chairman of the Senate Foreign Relations Committee, who also will not run for reelection in 2018, told reporters in assessing Trump's nine-month tenure: "I've seen no evolution in an upward way. As a matter of fact, it seems to me it's almost devolving."

With their distress calls, Flake and Corker joined a chorus of mainstream political leaders newly emboldened to excoriate Trump. Last week, former presidents George W. Bush, a Republican, and Barack Obama, a Democrat, both indirectly rebuked Trump's deportment and warned of peril for the nation under his watch, as did Sen. John McCain (R-Ariz.), who thundered about the rise of what he called "half-baked, spurious nationalism."

In his first response, Trump fired back with tweets early Wednesday characterizing Flake and Corker as bitter GOP outsiders who had "zero chance" of reelection. Trump also pointed to what he called a "love fest with standing ovations" by other Republican senators at a meeting Tuesday to discuss the party's tax plan.

"The reason Flake and Corker dropped out of the Senate race is very simple, they had zero chance of being elected. Now act so hurt & wounded!" Trump wrote.

The raw candor from two retiring senators came on a day when Trump made a rare trip to the Capitol for an intended show of party unity, lunching privately with Republican senators to rally support for his plan to cut taxes.

For a Republican Party that has been riven by internal turmoil for nearly a decade, the Flake-Corker rupture with Trump exacerbated the ferocious war between the party's seasoned leaders and its anti-establishment forces, now rallying under the banner of Trumpism. Polls show that the overwhelming majority of Republican voters back Trump, and the fact that two of the president's most vocal critics in the Senate are retiring underscores how dangerous it is for politicians seeking reelection to break with the president and risk the wrath of his loyal supporters.

[Sen. Jeff Flake of Arizona will retire, citing direction of GOP under Trump]

Flake's 18-minute speech was perhaps the most sweeping indictment of Trump delivered by a Republican to date. Flake, 54, spoke with bewilderment and sadness, his voice cracking at times, about what he viewed as the withering of morality and civility in the national dialogue.

"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 and decency."

Flake added: "We must stop pretending that the degradation of our politics and the conduct of some in our executive branch are normal. They are not normal. Reckless, outrageous and undignified behavior has become excused and countenanced as 'telling it like it is' when it is actually just reckless, outrageous and undignified."

Some Republican elder statesmen who have been highly critical of Trump celebrated Flake's remarks and called on other elected Republicans to further distance the party from the president.

"Am I concerned about what we are supposed to do for the next three-plus years with this man in the White House? Yes, I'm very concerned," said John C. Danforth, a former Republican senator from Missouri and U.S. ambassador to the United Nations. "But the best I can think of right now is simply making it clear to the American people that the Republican Party is what it has been in the past, and that is not Donald Trump."

Still, Danforth said he is concerned that by giving up their seats, Flake and Corker are "leaving the field open" to insurgent candidates inspired by former White House chief strategist Stephen K. Bannon, who is leading a rebellion against establishment Republicans.

Already, a crop of Bannon-inspired conservative outsiders is emerging nationally. From Alabama to Mississippi to Nevada, these contenders are hoping to disrupt the 2018 midterm elections. They could determine whether the GOP maintains its narrow majorities in the Senate and the House — and whether Senate Majority Leader Mitch McConnell (R-Ky.) and House Speaker Paul D. Ryan (R-Wis.) hold their leadership positions.

Bannon claimed victory with Flake's departure. "Many more to come," he predicted in a text message to The Washington Post.

Andy Surabian, a Bannon associate who advises Great America Alliance, a pro-Trump political group, said: "This is a victory for President Trump and all of his supporters across the nation. Jeff Flake was America's top 'Never Trumper,' so getting his scalp is a signal to Never Trumpers everywhere that their time is up."

[’The president’s wingman’: Absent in the West Wing, Bannon stays close to Trump]

At the White House, press secretary Sarah Huckabee Sanders said Flake's and Corker's comments were "petty" and suggested that they were retiring because they could not win reelection. She boasted from the briefing room lectern that Trump was more popular in Arizona and Tennessee, both states he carried in 2016, than the two departing senators.

"The voters of these individual senators' states are speaking in pretty loud volumes," Sanders said. "I think that they were not likely to be reelected, and I think that shows that the support is more behind this president than it is those two individuals."

At the Capitol, meanwhile, Republican leaders reacted cautiously Tuesday, eager to offer support to their colleagues but fearful of breaking their fragile bonds with a president who has been quick to explode at personal slights.

Following Flake on the Senate floor, McConnell thanked him "for the opportunity to listen to his remarks" and honored the Arizonan, whom he called "a very fine man, a man who clearly brings high principles to the office every day."

Tuesday's thunderclap exposed the threadbare relationship Trump has with the GOP. At the closed-door lunch, Trump received a standing ovation from Republican senators. Yet for months, many of these lawmakers privately have seethed at the president's actions and language, as Flake and Corker did publicly in concluding that Trump is an unstable presence in American political life.

"This is the ice beginning to crack," said Peter Wehner, a Trump critic who has advised several past Republican presidents. "This is an extraordinary moment because the members of the president's own party know that he is not fit in some fundamental way to be the president. These views that they've kept in the shadows are now being exposed to the light."

Democrats balked at the notion that Republicans were somehow unsullied because two GOP senators rapped Trump.

"Unfortunately, Republicans in Congress remain in lockstep with the Trump agenda and silent in the face of the president's disgraceful behavior," Democratic National Committee Chairman Tom Perez said in a statement.

[The great dealmaker? Lawmakers find Trump to be an untrustworthy negotiator.]

Flake's Tuesday speech was the surprise culmination of more than a year of simmering criticism of Trump and the direction the party has taken under him. Flake published a best-selling book this summer, "Conscience of a Conservative," that chastised the president's character and ideology, stunning his colleagues and stoking Trump's anger. The president vowed to work to defeat him if he sought reelection in 2018.

Recent polls showed Flake trailing the leading Democratic Senate candidate, Rep. Kyrsten Sinema (Ariz.), as well as potential primary rivals.

The emerging Republican field is robust and includes Kelli Ward, a former Arizona state senator and hard-line conservative known for incendiary statements. Several members of the state's congressional delegation, including Reps. Martha McSally and David Schweikert, are also considering bids, according to Arizona Republicans.

Flake said Tuesday that he was not comfortable making the policy concessions on issues such as trade and immigration — or withholding criticisms of Trump's behavior — that he felt he would have to make to satisfy Republican primary voters galvanized by anger and grievance.

"Sustained incumbency is certainly not the point of seeking office, and there are times when we must risk our careers in favor of our principles," Flake said. "Now is such a time."

Flake, who once ran the Goldwater Institute, a libertarian think tank based in Phoenix and named for the late Republican senator Barry Goldwater, won a seat in the House in 2000 and served there for a dozen years before being elected to the Senate in 2012.

As he spoke on the Senate floor, a handful of colleagues from both parties sat grimly across the chamber. When he concluded, Sen. Ben Sasse (R-Neb.), a sometime critic of Trump, clapped loudly, prompting others to stand and applaud.

McCain spoke a few minutes later and called his colleague "a man of integrity and honor and decency and commitment," saying Flake's service to the country is "one of honor, of brilliance and patriotism and love of country."

Asked later by reporters if he thought other senators should speak in the same terms about Trump as Flake had, McCain demurred. "It's up to every senator," he said. "It's not up to me."

[’Utterly untruthful’: Corker unbound as he attacks Trump over fitness, competence]

Corker unburdened himself Tuesday of his feelings about Trump — first in a trio of television morning show interviews, then in a tweet and then in a hallway gaggle with reporters.

Corker said Trump was "utterly untruthful" and called him "the L-word"; expressed hope that he would stand down to let Congress formulate a tax plan without him; said he should "leave it to the professionals" to handle the North Korea nuclear crisis; said he was not a role model for children; and urged West Wing aides to "figure out ways of controlling him." Corker also said he would not support Trump for election again.

The succession of brittle comments seemed to enrage Trump, who responded with several tweets, calling the short-statured senator "liddle" and "a lightweight," as well as "incompetent."

Republican leaders were also swift to dismiss Corker's comments as mere distractions, insistent upon showcasing party cohesion despite the evident disorder.

"All this stuff you see on a daily basis — Twitter this and Twitter that? Forget about it," Ryan told reporters.

"There's a lot of noise out there," McConnell said. "We have a First Amendment in this country; everybody gets to express themselves. But what we're concentrating on is the agenda the American people need."

Read more at PowerPost

Ed O'Keefe, Ashley Parker, Elise Viebeck, David Weigel and Brian Murphy contributed to this report.