Chairman Chuck Grassley is accusing Fusion GPS founder Glenn Simpson of giving “extremely misleading” statements during August 2017 testimony to the Senate Judiciary Committee — and in doing so is comparing it to the false testimony that former Trump lawyer Michael Cohen also gave to Congress at the same time.

In a Monday letter to Sen. Richard Blumenthal, D-Conn., Grassley accuses Simpson — the co-founder of political research firm Fusion GPS — of giving “extremely misleading if not outright false testimony” when he gave a closed-door deposition on Aug. 22, 2017.

“So you didn’t do any work on the Trump matter after the election date, that was the end of your work?” Simpson was asked in his August 2017 deposition by a Republican committee staffer.

“I had no client after the election,” said Simpson.

“As we now know, that was extremely misleading, if not an outright lie,” Grassley said in his letter to Blumenthal.

Fusion GPS was hired in April 2016 on behalf of Hillary Clinton’s campaign and the Democratic National Committee to perform opposition research on then-candidate Donald Trump.

In June 2016, Fusion GPS retained Christopher Steele, a former British spy, to research any Russian connections to Trump. He then produced a series of memos between June and December 2016 connecting Trump to Russia, becoming what is now known as the salacious and unverified Steele dossier.

In his letter, Grassley noted that former Senate staffer Daniel Jones told the FBI in March 2017 that he hired Fusion GPS and Steele after the 2016 election to continue an investigation into Trump’s possible ties to Russia.

Jones, who once worked for the committee’s top Democrat Sen. Dianne Feinstein of California, told the FBI that he “had secured the services Steele, his associate [redacted], and Fusion GPS to continue exposing Russian interference in the 2016 Presidential election.”

Jones told the FBI that the group leading the research “was being funded by 7 to 10 wealthy donors located primarily in New York and California, who provided approximately $50 million.”

The end goal was to pass information to lawmakers, the FBI and the press, Jones told the FBI.

“Contrary to Mr. Simpson’s denial in the staff interview, according to the FBI and others, Fusion actually did continue Trump dossier work for a new client after the election,” wrote Grassley to Blumenthal. “So, despite the fact Mr. Simpson said he had no client after the election, he in fact did, and that client revealed himself to the FBI.”

The letter comes in response to Blumenthal’s letter accusing Donald Trump Jr. of lying about his statements to the Senate Judiciary Committee in September 2017. Blumenthal wrote a letter to Grassley on Monday regarding the accusations after Cohen pleaded guilty last week to lying to congressional investigators as part of special counsel Robert Mueller's investigation into Russian election interference.

[READ: Michael Cohen's plea agreement]

Grassley reminded Blumenthal that he had already told another of the committee’s Democratic members about Simpson’s inconsistent testimony — Sen. Chris Coons of Delaware in May — and that he would be rejecting his calls to interview Trump Jr. again.

In October, Simpson invoked his Fifth Amendment rights to avoid testifying before the House Judiciary and Oversight committees. Simpson testified to the House Intelligence Committee, and that transcript was released in January.