Senate Judiciary Committee Chairman Charles Grassley is suggesting Fusion GPS co-founder Glenn Simpson uttered an “outright lie” when he testified to panel staffers. Mr. Grassley, Iowa Republican, made the charge in a letter to a Democratic senator who wanted Donald Trump Jr. recalled as a witness. Fusion is one of the key players in the FBI’s nearly two-year Russia investigation. It was Mr. Simpson, a private investigator, who tapped Democratic Party money to hire ex-British spy Christopher Steele, who produced the discredited anti-Trump dossier. In August 2017, Mr. Simpson testified in private to committee staff. Sen. Dianne Feinstein of California,...