A number of people whom the president has employed, defamed and then fired have slunk back to their master’s hand like beaten dogs. In John Bolton, President Trump has finally met his counterpunching match. Even as Trump’s defenders in the Senate trial were denying the factual basis for impeachment, leaks from the former national security adviser’s book have confirmed it. Senate Republicans can no longer deny Trump’s quid pro quo of military aid in exchange for slander without being deceptive or delusional.

Bolton’s bombshell comes on top of a comprehensive, well-argued case for conviction by House impeachment managers. Their presentation of the evidence, their version of constitutional interpretation and their appeal to senatorial self-respect were all compelling. The response of Republicans — putting fingers in their ears and loudly humming “Battle Hymn of the Republic” — has been less impressive.

If Republican senators refuse to call new witnesses (including Bolton) under these circumstances, there would be only one plausible explanation: They fear Trump’s retribution. Such fear, of course, is justified.