WASHINGTON — Senator Dianne Feinstein of California, the top Democrat on the Judiciary Committee, on Tuesday unilaterally released a highly anticipated transcript of the committee’s interview with one of the founders of the firm that produced a salacious and unsubstantiated dossier outlining a Russian effort to aide the Trump campaign.
The interview, with Glenn R. Simpson of Fusion GPS, took place last summer and was expected to shed light on the origins of the firm’s work, its concerns about the Trump campaign’s activities, and what the F.B.I. may have done with the information.
Mr. Simpson and Peter Fritsch, the firm’s co-founders, had called for the transcript’s release last week in an op-ed in The New York Times in which they accused Republicans of concealing their testimony in an effort to smear the dossier and distract from questions about the Trump campaign’s ties to Russia.
In the op-ed, Mr. Simpson and Mr. Fritsch said the transcripts would show that they told lawmakers that they believed the dossier had been taken seriously by the F.B.I. because it matched reports that the bureau was receiving from other sources.
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.