To believe most media descriptions of Justice Department lawyer Bruce Ohr, he is a nonentity, unworthy of the attention President Trump has given him. This is remarkable, given that Mr. Ohr spent Tuesday confirming for Congress its worst suspicions about the Federal Bureau of Investigation’s abuse of its surveillance and sourcing rules.

If Mr. Ohr is only now under the spotlight, it’s because it has taken so much effort to unpack his role in the FBI’s 2016 investigation of the Trump campaign. Over the past year, congressional...