"I rise today to say, 'Enough,'" he told his colleagues, ripping the president of his party for policies and behavior he called "dangerous to a democracy."
Last month, Flake delivered a second Senate floor speech likening Trump's words to those of 20th century Russian dictator Josef Stalin. As he leaves his Senate seat, he has pointedly declined to rule out challenging Trump by running for president himself.
Yet he has not waged an all-out assault on the president's agenda. He joined fellow Republicans in delivering Trump's greatest victory so far, backing the $1.5 trillion tax cut despite his own expressed concerns about budget deficits.
Flake sat down to discuss his approach to challenging this White House, and his plans, in the Senate office that doubles as his living quarters when he's in Washington. What follows is a condensed, edited transcript of our conversation.
JOHN HARWOOD: You going to have a hard time giving up this bedroom, home sweet home?
SEN. JEFF FLAKE: Going to have to. I won't miss it.
HARWOOD: Have you done that the whole time you've been in Congress?
FLAKE: I'm cheap and poor. And even if I wasn't, I'd do it anyway. It's just easier.
HARWOOD: You are the former head of the Goldwater Institute.
HARWOOD: I saw a copy of "Conscience of a Conservative" over there. And I think you've always had a reputation as a person of principle in the Congress. There are lots of different kinds of principles. What are the most important ones to you at this moment?
FLAKE: Fiscal discipline, limited government. Being a conservative, I've always felt, was believing in limited government, economic freedom and individual responsibility.
HARWOOD: But there are ideological principles, there are principles of character, there are principles of national security. You've talked about threats to constitutional democracy and echoes of Josef Stalin. How important is that in your hierarchy right now?