Sen. Chuck Grassley Charles (Chuck) Ernest GrassleyGOP lawmakers distance themselves from Trump comments on transfer of power The Hill's 12:30 Report: Ginsburg lies in repose Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy MORE (R-Iowa) is threatening to subpoena a firm tied to the controversial opposition research dossier on President Trump.

Grassley, who chairs the Judiciary Committee, sent a letter to Glenn Simpson, a former Wall Street Journal reporter who now heads Fusion GPS, warning that if he refused to "comply voluntarily" with the committee's request, senators "will begin consideration of compulsory process under its rules."

"Your attorney has failed to sufficiently explain your claims that responding to the Committee’s requests would infringe upon or violate your and/or Fusion’s First Amendment rights, attorney-client and attorney work product privileges, and confidentiality agreements," Grassley wrote in a letter released on Friday.

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Grassley is giving Simpson until Wednesday to respond to his letter and hand over "responsive documents."

The back-and-forth comes as Grassley digs deeper into former British spy Christopher Steele's controversial dossier, which alleges that the Russians have compromising information on Trump.

Grassley sent a letter to Fusion GPS in March requesting information on the dossier, as well as details on Steele's hiring, and asked if it had shared information with the FBI.

But lawyers for the firm said on April 9 that the letter "calls for information and documents protected by the First Amendment rights, attorney-client privilege, attorney work product, and contractual rights (e.g., confidentiality agreements) of Fusion GPS and/or its clients."

Grassley argued that the firm hasn't been able to give a "clear explanation of the basis for the claimed privileges and rights."