“We must never adjust to the coarseness of our dialogue, with the tone set up at the top,” Flake said. “We must never accept the deadly sundering of our country. The personal attacks, threats against principles and freedoms and institutions, and flagrant disregard for decency.”

“Reckless, outrageous and undignified behavior has become excused as telling it like it is when it is actually just reckless, outrageous and undignified,” Flake continued. “And when such behavior emanates from the top of our government, it is something else. It is dangerous to a democracy.”

Flake’s criticism of the president had damaged his standing with the GOP base without winning over Democrats or independents, and his poll numbers had been in decline since the beginning of the year. Public polls had him trailing former state Sen. Kelli Ward in a GOP primary and Democratic Rep. Kyrsten Sinema in a general election.

Ward is an archconservative who recently won the endorsement of former White House strategist Steve Bannon, but mainstream Republicans consider her unelectable in a general election. Sinema, a progressive-turned-moderate with an inspiring life story and an impressive campaign war chest, is the likely Democratic candidate in the contest.

“Steve Bannon adds another scalp to his collection,” said Andy Surabian, a senior adviser to the pro-Trump nonprofit Great America Alliance and a top ally of Bannon’s.

The White House also indicated it wasn’t sad to see one of Trump’s most prominent GOP adversaries leave the Senate.

“I haven’t spoken with him directly since the announcement by Sen. Flake, but I think that, based on previous statements and certainly based on the lack of support that he has from the people of Arizona, it’s probably a good move,” White House press secretary Sarah Huckabee Sanders said when asked for Trump’s reaction to Flake’s decision.

Establishment Republicans said they hope Flake’s retirement will increase their chances of defeating Ward, allowing them to unite behind an establishment candidate without a history of attacking the president, who remains sacrosanct with GOP primary voters.

It’s not clear who else might enter the GOP primary, though operatives mentioned state Treasurer Jeff DeWit, Rep. Martha McSally and Rep. David Schweikert as possibilities who could unite both wings of the party. One top Republican strategist said Gov. Doug Ducey would likely play a leading role in finding a candidate.

Flake refused to endorse Trump during the 2016 election and wrote a book criticizing the president as insufficiently conservative. The continued potshots at the president eroded Flake’s own support among GOP primary voters, but he refused to back down and had begun hiring campaign staff, raising additional money and preparing his reelection bid.

“We’ve taken a banner that is not familiar to us as Republicans. And I don’t know how long this will last,” Flake told POLITICO in an interview last week, referring to the Trump-led GOP.

Flake informed Sen. John McCain (R-Ariz.) of his decision earlier Tuesday. “I knew that he was thinking about it but very sorry it’s happened,” McCain said. “He’s one of the most honorable men I’ve ever known.”

Democrats echoed that sentiment, speaking to Flake’s popularity in the Senate among members of both parties.

“He is one of the finest human beings I’ve met in politics. He is moral, upright and strong, and he will be missed by just about everybody in the Senate,” Minority Leader Chuck Schumer (D-N.Y.) said in a statement.

Flake’s retirement upends what was pegged as one of the most closely watched Senate races in 2018, but it’s unclear exactly what the effect will be.

Steven Law, president of the Mitch McConnell-aligned Senate Leadership Fund, predicted that Flake’s departure would guarantee Ward’s defeat.

“The one political upshot of Sen. Flake’s decision today is that Steve Bannon’s hand-picked candidate, conspiracy theorist Kelli Ward, will not be the Republican nominee for this Senate seat in 2018,” Law said.

A top Republican strategist said Flake’s team had been polling the race since the beginning of the year and had repeatedly found no path to victory in either the GOP primary or the general election. About a month ago, Flake’s team delivered that news to the senator, the strategist said, and Flake had been considering retirement ever since.

“He basically lost to all comers,” the GOP strategist said. “There wasn’t a Republican or a Democrat you could put next to him on the ballot who he wouldn’t lose to.”

Flake’s major problem was with the center of the electorate. Independents were either unfamiliar with Flake or disliked him. While McCain shared Flake’s problems with the right wing of his own party, independents adored McCain, helping him easily win reelection last year over then-Rep. Ann Kirkpatrick.

If Flake’s floor speech — and retiring Republican Sen. Bob Corker’s scathing criticism of the president earlier Tuesday — was any indication, Trump could be in for a long year of vocal opposition from prominent members of his own party before they leave the Senate.

“I’m aware that there’s a segment of my party that believes that anything short of complete and unquestioning loyalty to a president who belongs to my party is unacceptable and suspect,” Flake said. “The notion that one should stay silent as the norms and values that keep America strong are undermined, and as alliances and agreements that ensure the stability of the entire world are routinely threatened by the level of thought that goes into 140 characters — the notion that we should say or do nothing in the face of such mercurial behavior — is ahistoric and profoundly misguided.”

Seung Min Kim and Elana Schor contributed to this report.