A top Democrat on the Senate Judiciary Committee said Thursday that opposition research firm Fusion GPS can likely be compelled to reveal the political clients who hired it to investigate Donald Trump during the presidential campaign.

Fusion GPS has refused requests by the committee to identify who hired it to investigate Trump, but Connecticut Sen. Richard Blumenthal said in an interview on MSNBC’s “Morning Joe” that the firm could be compelled to name its clients through subpoena. (RELATED: The Search For Fusion GPS’ Mysterious Republican Client)

“Should the government be able to compel an investigative firm to reveal who its clients were?” MSNBC contributor Mark Halperin asked Blumenthal.

“An investigative firm, unless they are working for an attorney and somehow that kind of of investigative result is a attorney-client work produce, certainly is unprotected. So the committee ought to be able to subpoena all …,” he said, before Halperin interrupted.

According to news reports, an anti-Trump Republican donor hired Fusion GPS in September 2015 to investigate Trump. By the following June, an ally of Hillary Clinton’s was paying Fusion GPS to investigate Trump’s activities in Russia. Fusion GPS former British spy Christopher Steele to conduct the investigation, which resulted in the uncorroborated dossier.

The Judiciary Committee has been the most aggressive of the three congressional panels investigating Russia-related matters in trying to find out who hired Fusion GPS.

Iowa Sen. Chuck Grassley, the Republican chairman of the committee, has questioned whether it is appropriate that the FBI has used research compiled by political opponents of Trump as part of its investigation into possible collusion between the Trump campaign and Russian government.

Despite demands from the Judiciary Committee, Fusion GPS and its co-founder, Glenn Simpson, have showed no indication that they clients will be revealed.

Fusion GPS has claimed to be protected by the First Amendment, confidentiality clauses, and attorney-client privilege. And on Tuesday, Simpson, a former Wall Street Journal reporter, declined to identify his clients during a 10-hour interview with Senate Judiciary Committee staffers. (RELATED: Fusion GPS Founder Refuses To Identify Client In Marathon Senate Session)

Josh Levy, a lawyer for Simpson, released a statement following the marathon interview sessions saying that the opposition researcher did not reveal his clients. Levy also said that Simpson stands by the dossier, which BuzzFeed News published on Jan. 10.

“Did he, in fact, refuse to say who his clients were in the testimony?” Halperin asked Blumenthal of Simpson’s remarks in the interview.

“I can’t go into the substance of the testimony was. again, my hope is that there will be no objection to his testifying fully and frankly before our committee,” Blumenthal said.

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