Senate Judiciary Chairman Chuck Grassley Charles (Chuck) Ernest GrassleySenate Republicans signal openness to working with Biden Senators offer disaster tax relief bill Trump spikes political football with return of Big Ten season MORE (Iowa-R) is pushing back on Fusion GPS, the opposition research firm that helped assemble the controversial "Steele dossier" about President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE.

The co-founders of Fusion GPS, Glenn Simpson and Peter Fritsch, wrote a blistering op-ed for The New York Times on Tuesday slamming the GOP's investigations into the dossier. They accused Republicans of selectively leaking details of their testimony to the press and demanded that full transcripts be released.

But Grassley, whose panel interviewed Simpson in August, in a statement Wednesday said Simpson has already been invited to testify publicly about the dossier, adding that the offer "remains on the table."

ADVERTISEMENT

A spokesman also said that it was Simpson who demanded that the transcript of his testimony to the Judiciary Committee be kept private.

"Senator Grassley provided Fusion GPS an opportunity for transparency six months ago when he invited the firm to publicly testify at an open committee hearing. Mr. Simpson declined," said Grassley spokesman Taylor Foy.

"When faced with a subpoena from the Chairman and Ranking Member, Mr. Simpson refused to provide public testimony, using his Fifth Amendment right against self-incrimination to negotiate for a closed-door interview," he added.

"Despite his public statements, Mr. Simpson and his attorney demanded during the interview that the transcript be kept confidential. Mr. Simpson has refused to answer dozens of questions voluntarily, and has failed to provide the Committee with documents and responses to follow-up questions after the interview," it continued.

Simpson appeared before the Judiciary Committee in August for a 10-hour closed-door interview, after reaching an agreement to talk to the panel on a voluntary basis as part of its ongoing Russia probe.

ADVERTISEMENT

He similarly appeared before the House Intelligence Committee in mid-November for a voluntary, closed-door interview in which he maintained the right decline to answer the panel's questions.

Simpson and Fusion GPS have come under heavy scrutiny for their role in the Russian dossier, which former British intelligence officer Christopher Steele compiled.

In the op-ed, Simpson and Fritsch said they hired Steele to look into why Trump repeatedly sought deals in Russia, "a notoriously corrupt police state that most serious investors shun."

"What came back shocked us. Mr. Steele’s sources in Russia (who were not paid) reported on an extensive — and now confirmed — effort by the Kremlin to help elect Mr. Trump president," they wrote.

Republicans are interested to learn who Steele’s sources were as well as how the memos have been used in the FBI's investigation of Russia's election interference.

ADVERTISEMENT

Democrats, meanwhile, say that their GOP colleagues are trying to discredit Steele and argue that it is more important to investigate whether dossier's assertions about Trump are true.

Some of the allegations in the dossier have been shown to be false, while others have either been supported by public evidence or remain unproven.

The funding of the dossier has also been closely scrutinized.

Reports revealed that Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE's campaign and the Democratic National Committee (DNC) funded Steele's opposition research. The Washington Free Beacon, a conservative website, has also acknowledged that it hired Fusion GPS, before Steele was involved, to look into Trump during last year's Republican presidential primary.