Mr. Simpson has also spoken with investigators from the House and Senate intelligence committees, which have their own investigations into Russia’s election interference. Neither committee had intentions of releasing the transcripts from its interview with Mr. Simpson.
Ms. Feinstein moved to do so without the committee’s Republican chairman, Senator Charles E. Grassley of Iowa, almost certainly escalating partisan tensions on the committee. Mr. Grassley last week rejected the firm’s request to release the transcript, citing concerns that its release could taint the committee’s ongoing investigations related to Russia’s election meddling and the F.B.I.’s response. He invited Mr. Simpson to instead testify publicly.
Mr. Grassley, along with another Republican on the panel, Senator Lindsey Graham of South Carolina, issued a criminal referral last week for the dossier principal author a former British spy named Christopher Steele. The senators said they had reason to believe Mr. Steele had lied to federal authorities about his contacts with the media and urged the Justice Department to investigate. Democrats called the referral a partisan distraction.
Fusion GPS was first hired by The Washington Free Beacon, a conservative website, in September 2015. And the Democratic National Committee and the Clinton campaign later funded its work, including that of Mr. Steele.
Republicans have repeatedly and vocally raised concerns that the dossier — a set of reports paid for by Democrats — could have been mishandled by the F.B.I. as it was opening its own investigation into the Russian effort and the Trump campaign.