But Mr. Flake, choosing the Senate floor for his fierce denunciation of the president, appeared to issue a direct challenge to his colleagues and his party.

“It is often said that children are watching,” he said. “Well, they are. And what are we going to do about that? When the next generation asks us, ‘Why didn’t you do something? Why didn’t you speak up?’ What are we going to say?”

Without mentioning Mr. Trump by name, Mr. Flake, 54, took direct aim at the president’s policies, notably his isolationist tendencies, but also his behavior and that of his aides. In his time in Washington, Mr. Flake embodied an old-line conservatism. He avidly pitched smaller government, spending cuts and an end to home-district pork-barrel projects, but also supported free trade, engagement with the world and an openness to immigration.

Those positions stood in marked contrast to Mr. Trump’s inward-looking, anti-immigration nationalism. The senator had already touched on such themes in a book he published in August, “Conscience of a Conservative,” that was highly critical of the president. In his speech, he was at turns somber and passionate.

“We must stop pretending that the degradation of our politics and the conduct of some in our executive branch are normal,” Mr. Flake said. “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. And when such behavior emanates from the top of our government, it is something else. It is dangerous to a democracy.”