President Donald Trump boosted a tweet Monday promoting a controversial allegation from an indicted Ukrainian oligarch: that a top U.S. diplomat put fabricated information about the mogul in a diplomatic cable.

That diplomat happens to be one of Democrats’ key impeachment witnesses. And that oligarch happens to have a long-standing beef with Joe Biden.

Scott Adams, a Washington, D.C., talk radio host, sent out a tweet Monday night about U.S. Ambassador to Ukraine Bill Taylor, who delivered some of the impeachment inquiry’s most damaging testimony yet. The tweet alleged that Taylor lied about Ukrainian natural gas baron Dmytro Firtash in a cable to State Department headquarters in 2008. At issue was a conversation Taylor had with Firtash in Kyiv that December. Taylor wrote in a diplomatic cable (later published by WikiLeaks) that Firtash told him he had “acknowledged ties to Russian organized crime figure [Semion] Seymon Mogilevich,” one of the most notorious accused mobsters on the planet. According to Taylor, Firtash said “he needed Mogilevich's approval to get into business in the first place,” but had not committed any crimes in the course of his business.

When WikiLeaks published the cable in 2010, Firtash issued a statement on his website disputing its contents. Firtash, the statement claimed,“has never stated, to anyone, at any time, that he needed or received permission from Mr. Mogilevich to establish any of his businesses.”

Earlier this year, Firtash reiterated that defense. Without mentioning any American official by name, he said someone must have fabricated the detail about Mogilevich. Taylor, meanwhile, has defended the State Department’s notes.

The Justice Department appears to side with Taylor; its lawyers have argued in court that Firtash has ties to Russian organized crime. The criminal charges he faces, however, don’t involve any such alleged relationships. Instead, the Justice Department charged him in 2014 with helming a conspiracy to bribe Indian government officials.

Trump’s retweet, however, offers a presidential thumbs-up to Firtash’s side of the story, and raises a new line of attack on Taylor’s credibility for the president’s allies.

Asked about his sourcing for the allegations against Taylor, Adams told The Daily Beast, “My sources are solid Foggy Bottom people.” He also noted the explanation for the cable that Firtash provided to The Daily Beast earlier this year.

This specific defense of Firtash took hold in The Hill over the summer, when columnist John Solomon, whose articles informed Rudy Giuliani’s Biden-Ukraine investigation, published a piece in July claiming that Special Counsel Robert Mueller’s deputy said Firtash’s criminal charges in the U.S. might “go away” if he shared damaging information about Trump with Mueller’s team. Solomon cited “multiple sources with direct knowledge” and contemporaneous memos. Firtash and Solomon share the same lawyers: Victoria Toensing and Joe diGenova. The husband-wife team are veterans of the conservative movement’s most contentious legal battles, with longstanding ties in the Justice Department and Trump administration.