The Senate Judiciary Committee is subpoenaing the co-founder of the firm tied to a controversial opposition research dossier on President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE to testify before the committee next week.

Sens. Chuck Grassley Charles (Chuck) Ernest GrassleyGOP set to release controversial Biden report McConnell locks down key GOP votes in Supreme Court fight Senate Republicans face tough decision on replacing Ginsburg MORE (R-Iowa) and Dianne Feinstein Dianne Emiel FeinsteinNames to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-Calif.) said the move came after Glenn Simpson, the co-founder of Fusion GPS, declined to speak before the committee on Wednesday.

"A subpoena has been issued to compel his attendance. Simpson’s attorney has asserted that his client will invoke his Fifth Amendment rights in response to the subpoena," the Senate leaders said in a statement.

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Simpson had been listed as a witness for a public Judiciary Committee hearing scheduled for Wednesday regarding oversight of the Foreign Agents Registration Act and "attempts to influence U.S. elections."

But a spokesman for Grassley said earlier Friday that they had not heard from Simpson, who was not expected to agree to testify.

Donald Trump Jr. and Paul Manafort, Trump's former campaign manager, were also listed as witnesses for the hearing. They have also not confirmed they will testify.

Grassley and Feinstein said Friday while they were not issuing subpoenas for the two, they "reserve the right to in the future."

"Both Donald Trump Jr. and Paul Manafort ... have agreed to negotiate to provide the committee with documents and be interviewed by committee members and staff prior to a public hearing," Grassley and Feinstein said.

Under committee rules, the committee can subpoena individuals by a vote of the full committee or when Grassley and Feinstein — as the committee's top two members — have an agreement.

Lawmakers have been clamoring to hear from Trump Jr. over his recently revealed meeting last year with a woman that represented herself as a Russian government lawyer offering damaging information on then-Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE.

Manafort and Jared Kushner, Trump's son-in-law and senior adviser, were also in the meeting.

Kushner is reportedly meeting with the Senate Intelligence Committee next week.

Both panels are investigating Russia's election interference and any potential ties between the Trump campaign and Moscow.