He is their emperor, and they are his political Praetorian Guard.

A second thing that stands out from Mr. Cohen’s testimony is that the Republican Party has been as corrupted by its association with Mr. Trump as Mr. Cohen was by his. As Mr. Cohen told Republican lawmakers, “I did the same thing that you’re doing now. For 10 years. I protected Mr. Trump for 10 years.”

He then issued this warning to them: “The more people that follow Mr. Trump — as I did blindly — are going to suffer the same consequences that I’m suffering.” Mr. Cohen later explained the ethos of Trumpworld: “Everybody’s job at the Trump Organization is to protect Mr. Trump. Every day most of us knew we were coming and we were going to lie for him about something. That became the norm.”

The ethic that became the norm at the Trump Organization —- defacing the truth and disfiguring reality in the service of Donald J. Trump — is the ethic that has become the norm of the Republican Party and the American right.

This is what some of us who are conservatives and who have been lifelong Republicans have warned since Mr. Trump began his quest for the presidency — that his corruptions would eventually become theirs.

It didn’t take long.

The way these things happen is simple and insidious. In this case, because Mr. Trump was their party’s nominee, many Republicans felt duty bound to defend him, even though they would from time to time call him out for his worst offenses. They also held out the hope that Mr. Trump would grow in office and become more presidential.

What happened is quite different: As Mr. Trump was elected and then inaugurated, Republicans became more and more reluctant to call him out and more and more vocal in defending him and attacking his critics; rather than weakening, their loyalty to him intensified . And the president, rather than becoming more responsible, has become less restrained, more volatile, more unhinged. The result is the ethical wreckage we saw on display Wednesday.

Republicans should brace for even more damaging revelations. The evidence presented on Wednesday was harmful to the president, but Mr. Cohen quite likely revealed only a small fraction of what the Southern District of New York and the Mueller investigation have amassed. Mr. Cohen did suggest that federal prosecutors are investigating unspecified criminal allegations involving the president that have not been made public.