Washington pile-ons are never pretty, but this week’s political setup of Attorney General William Barr is disreputable even by Beltway standards. Democrats and the media are turning the AG into a villain for doing his duty and making the hard decisions that special counsel Robert Mueller abdicated.

Mr. Barr’s Wednesday testimony to the Senate Judiciary Committee was preceded late Tuesday by the leak of a letter Mr. Mueller had sent the AG on March 27. Mr. Mueller griped in the letter that Mr. Barr’s four-page explanation to...