Sen. Jeff Flake’s (R-AZ) shocking decision to retire is the latest blow to Republicans willing to stand up to President Donald Trump — and leaves the Senate reeling and the race to replace him in utter chaos.

Flake, the GOP’s loudest critic of the president, admitted he couldn’t win the Republican primary in an emotional speech on the Senate floor before warning his Republican colleagues that they were being “complicit” in allowing Trump’s “reckless, undignified and outrageous behavior” to continue.

But his decision to leave rather than stay and fight, coupled with the retirement of Sen. Bob Corker (R-TN), another top Trump GOP critic, and Sen. John McCain’s (R-AZ) ongoing serious health problems, mean the Senate is likely to see a dwindling of Republicans willing to stand up to Trump after the 2018 elections — and it’s unclear whether it improves Democrats’ slim prospects of winning back Senate control next election.

The mood for many senators was one of shock and sorrow, almost as if a member had died, with members of both parties visibly upset at the news.

“It’s just tragic. Flake is a completely honest broker,” Sen. Tim Kaine (D-VA) told TPM as he exited the Senate floor after Flake’s retirement speech. “When somebody who’s as good and decent a person as Jeff Flake does not feel like he can continue on it’s a very tragic day for the institution, and I felt the same way with the Corker announcement [last month]. … It’s a very depressing, very depressing day.”

“I’m surprised and I’m very saddened,” Sen. Chris Coons (D-DE) told TPM.

Corker lamented Flake’s decision as well.

“Flake is one of the best people in the United States Senate, no question, and the Senate will be less well off without him,” he said, saying Flake stood up to the “demagogues” and for American values.

But with the pair gone and McCain’s future in the Senate uncertain, it’s not clear who will fill that role. Corker didn’t have an answer when a reporter asked him who will be left to stand up when the “people who stand up against the demagogues” head for the exits.

“I’ll answer that question later,” he said after a pause.

If a Republican wins the seat, it’s almost certain to be one that’s more friendly to the president than the antagonistic Flake. Unless Democrats win this race and pull off an upset in the race to replace Corker, the 2018 elections might end up further weakening the Senate as a brake on Trump’s worst impulses with the few principled Republicans heading for the exits. That’s true even if every one of the 10 Democratic incumbents in states Trump carried wins reelection next year.

Flake’s decision also throws the race for his seat into utter chaos. Flake had been trailing hardline conservative former state Sen. Kelli Ward (R) by huge margins in primary polls due to his ongoing war with Trump, and Republicans, including his own strategists, were greatly worried he’d either lose to a fatally flawed nominee or come out of the primary so wounded that he couldn’t defeat Rep. Kyrsten Sinema (D-AZ) in the general election.

National Republican Senatorial Committee Chairman Cory Gardner (R-CO) said Flake will be “severely missed.” And while he said Arizona was a “Republican state” that voted for Trump, he conceded the race would be “a focus” of his committee heading into 2018.

Arizona Republicans admit Flake’s decision scrambles the race, though they’re divided about whether it helps or hurts their chances to hold the seat. One said Flake was “pretty much doomed” in a head-to-head against Ward who strategists think would be a disaster as the nominee, and argued the move “gives us a better chance of holding the seat.”

But he said there’s no predicting what will happen now, with a number of politicians from former Gov. Jan Brewer (R) to Reps. Martha McSally (R-AZ), Trent Franks and Dave Schweikert (R-AZ) seriously eying bids of their own. Former Arizona Sheriff Joe Arpaio isn’t ruling out a run of his own.

“It throws the whole thing upside down. The dominoes will be far and wide. I think you have potentially half of the congressional delegation jump in the race,” said the strategist. “It could be a very crowded field with a lot of big names. … This changes the trajectory entirely.”

While Republicans are likely to duke it out in a costly, possibly nasty August primary, Sinema is likely to have the field to herself — and time to build upon the $4.2 million war chest she already has.

Sinema refused to discuss the dynamics of the race, while calling Flake “a man of great integrity and great character” in a brief statement to reporters.

Rep. Raul Grijalva (D-AZ) told TPM it “helps” because “the struggle for control” between pro- and anti-Trump forces within the GOP is “only going to get more intense” in an open GOP primary.

But other Democrats aren’t so sure.

“It changes the dynamic and we’re going to have to wait until the scenario shakes out.” Rep. Ruben Gallego (D-AZ) told reporters Tuesday night. “Everything’s up in the air.”

Trump’s team and allies celebrated Flake’s decision. White House Press Secretary Sarah Huckabee Sanders said it was “probably a good move,” while former White House Chief Strategist Steve Bannon’s website was more blunt:

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WINN https://t.co/XOT6IO0K3r — Breitbart News (@BreitbartNews) October 24, 2017

But they were about the only ones celebrating.

“These are times when we need Republican colleagues particularly who are willing to do the difficult things of standing up to a president who clearly is violating basic tenants of the Constitution every day,” Sen. Debbie Stabenow (D-MI) told TPM, saying she was concerned things would get worse and not better after the 2018 elections. “I’m worried about the country.”

This story was updated at 6:35 p.m.