Fusion GPS co-founder Glenn Simpson on Tuesday pleaded the Fifth following a GOP-issued subpoena to testify behind closed doors before two House committees.

Joshua Levy, a lawyer representing Simpson and the opposition research firm behind the “Steele dossier," said his client would not be participating in the House Judiciary and House Oversight and Government Reform committees' joint investigation, calling it a smear campaign that aims to undermine special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's Russia probe.

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Simpson's attorney said the Republican-led Judiciary Committee has turned a blind eye to “White House efforts to influence and interfere with the Justice Department’s investigation in this administration,” noting that it has gone after whistleblowers and Mueller.

Levy compared their efforts to McCarthyism.

“Like Sen. [Joseph] McCarthy, this committee has largely conducted its business in secret confidential interviews and depositions, binding witnesses and their counsels to silence, while the members walk outside to all of you and the media and the public and selectively leak from those interviews to tell you what they want you to hear," Levy added.

Levy, who said his client has cooperated with the investigations of three other congressional committees, has recently taken aim at the GOP lawmakers on the Judiciary and Oversight committees.

In a letter last week, Levy said Simpson will “invoke his constitutional rights not to testify under the First and Fifth Amendments of the Constitution” because this inquiry “is not designed to discover the truth.”

“Consistent with the September 27, 2018 letter we sent to you, Mr. Simpson, whose testimony is a matter of public record, will not be participating in a confidential deposition before this Committee,” lawyers for Simpson wrote to Judiciary Committee Chairman Bob Goodlatte Robert (Bob) William GoodlatteNo documents? Hoping for legalization? Be wary of Joe Biden Press: Trump's final presidential pardon: himself USCIS chief Cuccinelli blames Paul Ryan for immigration inaction MORE (R-Va.) in an electronic letter, according to a copy obtained by The Hill.

Rep. Mark Meadows Mark Randall MeadowsAnxious Democrats amp up pressure for vote on COVID-19 aid Pelosi hopeful COVID-19 relief talks resume 'soon' The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November MORE (R-N.C.), one of the two GOP lawmakers present for Tuesday's interview led by committee staff, said the panel will be having discussions in the “coming days” on whether they will move to hold Simpson in contempt for refusing to testify.

“Simpson has to have a reasonable apprehension of criminal prosecution in order to validly invoke the Fifth … but if not, you cannot just invoke the Fifth to avoid answering congressional questions,” Meadows told reporters.

He also connected Simpson’s decision to plead the Fifth as a way to “protect himself” from a potential crime he may have committed.

Simpson, a former journalist who later co-founded Fusion GPS, hired former British spy Christopher Steele to help compile a dossier on Trump that ended up making a series of allegations about the president's possible ties to Russia — some that have been verified and some that remain unsubstantiated.

The opposition research against Trump was funded in part by 2016 Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE's campaign — a flashpoint for Republicans, who have pointed to ties between Fusion GPS and top federal officials as evidence to support their claims that there was political bias against Trump among the top brass at the FBI and Department of Justice.

Some conservative House members have alleged that a contractor working for Simpson's firm, Nellie Ohr, could have passed the dossier on to her husband, Bruce Ohr, who worked as a top official with the Justice Department during the election.

Simpson is one of several witnesses tied to the dossier whom Republicans have sought to interview as part of their joint investigation. Congressional investigators are expected to interview Nellie Ohr on Friday.

Levy on Tuesday said there seems to be a double standard when it comes to congressional testimony.

“Even this Congress last year did nothing when members of the president’s staff invoked their privileges or outright just decided not to comply with subpoenas,” Levy said, pointing to former White House chief strategist Stephen Bannon and former national security adviser Michael Flynn.

“And so what are we to conclude?" he asked. "Unfortunately, if you are not one of the president’s men, these congressional Republicans will show utter disdain for your constitutional rights.”