Senator Jeff Flake is a conservative Republican from Arizona. I’m a Democrat from Delaware. We have opposed each other on nearly every vote for as long as we’ve served in the Senate. So his announcement on Tuesday that he won’t seek re-election in 2018 should be great news for me.

That’s one way to look at it — the senator is nothing more than my political opponent, someone whose loss is my gain.

But that’s not at all how I see things. I may disagree with Mr. Flake on policy, but I consider him an honorable man, a loyal friend and a valued colleague. His retirement is deeply troubling to me because he represents a principled and patriotic Republican Party, one that has long championed strong American leadership around the world, and one I now fear is falling apart.

That should scare all Americans. It sure scares me.

How did Mr. Flake, who served in Congress since 2003 and has been in the Senate since 2013, become such an outsider in his own party that he wouldn’t seek re-election after only one term? Over the past year, right-wing populists have mocked his principled independence, donors have funded his primary opponent and President Trump, hardly a conservative and only recently a Republican, has openly wished for his defeat.