Cleta Mitchell is a top campaign-finance lawyer in Washington, D.C. This year she’s also been the target of a political and media smear that reveals some of the nastiness at work in the allegations of collusion between the Trump campaign and Russia.

A partner at Foley & Lardner, Ms. Mitchell was astonished to find herself dragged into the Russia investigation on March 13 when Democrats on the House Intelligence Committee issued an interim report. They wrote that they still wanted to interview “key witnesses,” including Ms....