Fusion GPS co-founder Glenn Simpson won't testify before the Senate Judiciary Committee next week despite a request for voluntary testimony from the committee.

Simpson co-founded the political intelligence firm Fusion GPS in 2009. During the 2016 campaign, the firm hired former British spy Christopher Steele, who produced a now-infamous dossier alleging Russian intelligence's influence over President Trump.

The Senate Judiciary Committee said yesterday that Simpson would testify next week. The chairman of the committee, Sen. Chuck Grassley Charles (Chuck) Ernest GrassleySenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE (R-Iowa), sent a letter asking for information about the dossier to Fusion GPS in March, and in June threatened to subpoena Simpson if he did not comply with the request for voluntary testimony.

Grassley rejected claims from Simpson's lawyer in June that responding to the committee's request would violate Simpson's First Amendment rights.

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"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 in June.

"Based on the minimal and vague explanations your attorney has provided, the Committee cannot adequately assess your claims. Thus, we must presume that they are unfounded."

Grassley's main questions for Simpson involve who hired Fusion GPS to produce opposition research against Trump, and whether the firm shared the document with the FBI.

"When political opposition research becomes the basis for law enforcement or intelligence efforts, it raises substantial questions about the independence of law enforcement and intelligence from politics," Grassley wrote in March.

In June, Grassley warned Simpson that if he didn't "comply voluntarily" with the committee's request, senators "will begin consideration of compulsory process under its rules."

Spokespeople for Grassley and ranking Democrat Sen. Dianne Feinstein Dianne Emiel FeinsteinMcConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Intensifying natural disasters do little to move needle on climate efforts MORE (Calif.) didn't respond to Politico's request for comment on whether they would begin the subpoena process.