But what of the question that usually excites conspiracy theorists like Mr. Schiff: Cui bono—who benefits?

It’s impossible to know whether Russia’s DNC leaks accounted for Hillary Clinton’s defeat, though it hardly seems likely. Powerful circumstantial evidence, however, suggests Democrats believed that linking Messrs. Trump and Putin would actually be their ace in the hole. They still do. One conclusion recommends itself overwhelmingly on the basis of circumstantial evidence: President Obama, in his final days, used his authority over the intelligence agencies to advance this partisan narrative.

A congressman with a modest profile, Mr. Schiff has been working hard ever since to become the public face—not to say the Joe McCarthy—of this witch hunt.

This is his main chance. It’s supposedly in his wheelhouse because, 27 years ago, he led the Justice Department’s effort to put a sad sack FBI agent in jail over his affair with an equally pathetic Soviet agent.