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. Mitchell, who they claimed was “involved in or may have knowledge of third-party political outreach from the Kremlin to the Trump campaign, including persons linked to the National Rifle Association (NRA).”

Two days later the McClatchy news service published a story with the headline “NRA lawyer expressed concerns about group’s Russia ties, investigators told.” The story cited two anonymous sources claiming Congress was investigating Ms. Mitchell’s worries that the NRA had been “channeling Russia funds into the 2016 elections to help Donald Trump.”

Ms. Mitchell says none of this is true. She hadn’t done legal work for the NRA in at least a decade, had zero contact with it in 2016, and had spoken to no one about its actions. She says she told this to McClatchy, which published the story anyway.

Now we’re learning how this misinformation got around, and the evidence points to Glenn Simpson of Fusion GPS, the outfit that financed the infamous Steele dossier. New documents provided to Congress show that Mr. Simpson, a Fusion co-founder, was feeding information to Justice Department official Bruce Ohr. In an interview with House investigators this week, Mr. Ohr confirmed he had known Mr. Simpson for some time, and passed at least some of his information along to the FBI.

In handwritten notes dated Dec. 10, 2016 that the Department of Justice provided to Congress and were transcribed for us by a source, Mr. Ohr discusses allegations that Mr. Simpson made to him in a conversation. The notes read: “A Russian senator (& mobster)… [our ellipsis] may have been involved in funneling Russian money to the NRA to use in the campaign. An NRA lawyer named Cleta Mitchell found out about the money pipeline and was very upset, but the election was over.”

A spokesman for Adam Schiff, ranking Democrat on the House Intelligence Committee, says the “Minority did not speak with Mr. Simpson or Fusion GPS about this,” though he declined to disclose who named Ms. Mitchell. Our sources say they can’t remember Ms. Mitchell coming up in any of the documents collected or witness interviews conducted for the investigation. So how did Mr. Schiff get his tip? Fusion’s media friends? Mr. Ohr? The FBI? Fusion GPS and Mr. Simpson did not answer a request for comment.

Ms. Mitchell says the fallout for her goes beyond inconvenience and a false allegation. Mr. Schiff’s team in May sent her a letter demanding testimony and documents, though no one in Mr. Schiff’s office alerted her before naming her in an official document.

She received similar demands from Senate Intelligence Chairman Richard Burr, who wanted Ms. Mitchell to turn over records related to “the transfer of money, or anything of value” between her and several Russians. After Ms. Mitchell in May responded that she had no information related to any of those Russians and accused the committee of being duped by “Glenn Simpson & Co.,” she heard nothing more.

But social media attacks on her haven’t ended. “That allegation impugns my ethical integrity and professional reputation,” she says, one reason she’s calling for Mr. Simpson to be prosecuted for lying to a federal official.

The Russian collusion accusations ginned up by Fusion at the behest of a law firm working for the Clinton campaign haven’t been corroborated despite two years of investigations. But no one should forget the smears that they and their media mouthpieces peddled along the way.