The federal criminal code didn’t even exist when the Constitution was drafted, yet the framers were aware of the innumerable ways an executive could abuse or violate his trust. They were also aware that impeachment was a long-established remedy for such abuses.

In his authoritative 19th-century legal treatise, Justice Joseph Story of the Supreme Court explained that impeachable acts are by their nature impossible to define in advance. There are many “purely political” offenses that qualify, he wrote, “not one of which is in the slightest manner alluded to in our statute book.”

This view is not remotely controversial. And yet Alan Dershowitz, the Harvard law professor and self-professed liberal who has lately taken to defending President Trump against all comers, insists that everyone’s got it wrong. That includes, apparently, Mr. Dershowitz himself, who argued back in 1998 that “if you have somebody who completely corrupts the office of president and who abuses trust and who poses great danger to our liberty, you don’t need a technical crime.” Now, Mr. Dershowitz argues that to be impeachable, a president’s behavior must be, if not criminal, at least “crime-like”— an argument that could charitably be described as legal-like.

Perhaps it should come as no surprise that a president who described the Constitution as “like a foreign language” would cling to such a legally preposterous claim. But to be fair to Mr. Trump and his lawyers, it’s the only one left. The evidence amassed during the House impeachment proceedings — that he shook down a foreign government in the service of his own re-election campaign — is overwhelming, and Mr. Trump has yet to counter any of it with so much as a single piece of paper or word of testimony. He has given himself no option but to say, in effect: “Yeah, I did it. So what?”

This is the Trumpian equivalent of, “It’s a free country!” And yet as every grade-school child quickly discovers, that does not mean you can do whatever you want.