Democrats have attacked Attorney General-designate William Barr for a memo in which he argued against a legal theory some claim could support prosecuting President Trump for obstruction of justice. Mr. Barr argued that an exercise of the president’s constitutional authority—for instance, firing James Comey as director of the Federal Bureau of Investigation—cannot be construed as obstruction even if prosecutors believe he did so for improper reasons.

At his confirmation hearings, Mr. Barr rightly stood his ground. Critics should...