Arizona Republican Jeff Flake announced his decision to retire from the U.S. Senate on Tuesday, pairing the news with a blistering attack on President Trump.

“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,” Flake said on the Senate floor. “And when such behavior emanates from the top of our government, it is something else: It is dangerous to a democracy. Such behavior does not project strength—because our strength comes from our values. It instead projects a corruption of the spirit, and weakness.”

While Flake has been perhaps Trump’s bluntest critic among elected Republicans, his speech represents a significant elevation of his rhetoric against the president. The remarks come on the same day that Trump and Senator Bob Corker engaged in another fierce exchange, and just days after Senator John McCain, Flake’s fellow Arizonan, fired a fusillade of criticism against the president. The three men’s criticism of a president from their own party is nearly unprecedented, and yet it also underscores the weakness of their position: There’s a strong likelihood that by the halfway point of Trump’s presidency, none of them will be in office. Corker has already announced his retirement at the end of this term, while McCain is battling an aggressive cancer.

Flake’s decision not to run for reelection was, in fact, largely an acknowledgment of reality: He was unlikely to defeat a primary challenge ahead of the 2018 Senate election. He said that he doubted he could win while remaining committed to core beliefs like support for free markets and immigration.

“I have decided that I will be better able to represent the people of Arizona and to better serve my country and my conscience by freeing myself from the political considerations that consume far too much bandwidth and would cause me to compromise far too many principles,” Flake said.

Flake’s Senate floor speech, which he delivered in a nervous but deeply felt manner, came not long after he announced his retirement to the Arizona Republic. The address was a stinging critique of Trump, impeaching him for divisiveness, moral bankruptcy, and lack of leadership on the national and international stages. He also adopted the language of the self-described “Resistance” to Trump, warning against allowing Trump to be “normalized,” though he at no time used the president’s name.

“We must never regard as ‘normal’ the regular and casual undermining of our democratic norms and ideals,” he 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. … They are not normal.”

Flake, as my colleague McKay Coppins wrote in September, has already been a notable critic of Trump’s. A first-term senator, he previously served for 10 years in the U.S. House, where he developed a reputation as a principled fiscal and moral conservative. (He is a committed Mormon.) In other words, he gave Democrats little reason to cheer him, but also had little in common with Trump other than the ‘R’ behind their names. He never endorsed Trump’s presidential campaign, and was at times surprisingly critical of him.

But since Trump’s election, the acrimony between the two men has grown. Flake published a book, Conscience of a Conservative, that borrowed its title from a volume by another Arizona Republican, the late Barry Goldwater. “Presidential power should be questioned, continually,” Flake wrote. “That’s what our system of government, defined by the separation of powers, is all about. It shouldn’t matter whether the president belongs to my party or to another one.”

Trump, meanwhile, was all too happy to agree that attacking members of one’s own party was defensible. Trump called Flake “toxic” and met with prospective primary challengers, notably Kelli Ward, a far-right conservative who unsuccessfully challenged McCain in 2016. These White House attacks, as well as the overall Republican turn toward Trump, made Flake vulnerable. Coppins wondered earlier this fall, “Is there no longer a place in politics for someone like Jeff Flake?” At the time, Flake still seemed to be trying for reelection, but since then has seen the writing on the wall.

White House Press Secretary Sarah Sanders, responding to Flake’s announcement, told reporters, “Based on the lack of support that he has from the people of Arizona, it’s probably a good move.”

By helping to push Flake out, Trump is making a gamble: The seat could end up in the hands of a far more friendly Republican, but nominating a more fringe candidate could also give Democrats a chance at a pickup, and thus weaken Trump’s allies in Congress.

Flake’s speech on Tuesday was not only an attack on a Republican president, but a jolting challenge to his own Republican colleagues.

“I am aware that a segment of my party believes that anything short of complete and unquestioning loyalty to a president who belongs to my party is unacceptable and suspect,” he said. “Acting on conscience and principle is the manner in which we express our moral selves, and as such, loyalty to conscience and principle should supersede loyalty to any man or party.”

Such language will win Flake plaudits from Democrats—Senate Democratic leader Chuck Schumer immediately called him “moral, upright, and strong”—but may be met with less pleasure by the Republicans he was calling out. Senate Majority Leader Mitch McConnell promptly rose to thank Flake for his service and praise his character, but McConnell, who has been a frequent target for Trump’s ire but has chosen to try to work with him, made no mention of Flake’s challenge.

Thus the peculiar position of the Trump critics. As Flake mentioned, his retirement frees him up to speak his mind about Trump; Corker has demonstrated that the same is true of him. McCain, reaching the end of his life and career, has little to lose either. But Senator Lindsey Graham, a past Trump critic who might naturally align with these men, has in recent weeks been cozying up to the president. Whatever the critics gain in freedom to speak, they lose in power by making themselves lame ducks and then leaving office.

It is unclear whether their blunt talk will bring over colleagues—or, perhaps more precisely, will convince those colleagues to publicly air their private misgivings and criticisms of the president. With or without that, it is unclear what a small core of anti-Trump senators can do to impede the president. Flake praised James Madison and spoke of the obligation of the Senate to check an out-of-control president, but translating that into action is tough. Many establishment Republicans opposed Trump’s presidential candidacy, but failed to stop him.

Short of pushing for an impeachment of Trump—unlikely for many reasons, not least of which is GOP control of the House—what can Flake and his friends do? They could threaten to block all major legislation, but Trump’s inefficacy and the divisions within the Senate GOP caucus are so far accomplishing that on their own (although McCain was central in sinking the latest Obamacare-repeal effort). They might, for example, join with Democrats to force certain issues, or to demand that the Senate use regular order. Such a move would be shocking, perhaps more shocking even than the sorts of verbal critiques of Trump they are offering. Do the critics have the fortitude for such action? Can they find alternative methods? Flake has about one year and two months to figure that out.