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.