House Intelligence Committee Chairman Devin Nunes, a California Republican, in Washington on March 24. J. Scott Applewhite/AP

The founders of the opposition-research firm that produced the dossier alleging ties between President Donald Trump's campaign team and Russia will invoke constitutional privileges and decline to testify before the House Intelligence Committee, their attorney wrote in a letter obtained by Business Insider on Monday.

"We cannot in good conscience do anything but advise our clients to stand on their constitutional privileges, the attorney work product doctrine and contractual obligations," Fusion GPS' counsel Josh Levy wrote in response to subpoenas issued earlier this month by the committee's chairman, Rep. Devin Nunes.

Levy argued that complying with the subpoenas would violate "the First Amendment rights" of the founders — Glenn Simpson, Thomas Catan, and Peter Fritsch — "and would chill any American running for office ... from conducting confidential opposition research in an election."

"Should you compel any of our three clients to appear at the scheduled deposition, they will invoke their constitutional privileges not to testify," Levy wrote. "Since that will be the case, we ask that the Committee excuse them from appearing."

Nunes stepped aside from the committee's Russia investigation in early April following his decision to brief Trump and the press on classified intelligence without telling his fellow committee members. But he quickly began conducting his own investigation into "unmaskings" by the Obama administration and the credibility of the dossier.

Levy contended in the letter that Nunes' "unilateral issuance of these subpoenas violates your recusal and further undermines the legitimacy of this investigation." He pointed to the fact that Nunes had issued the subpoenas just 24 hours after Fusion's counsel met with "majority and minority staff" for approximately an hour to discuss "a way forward for voluntary cooperation."

"Based on this Committee's bad faith interactions with the undersigned counsel and its pattern of unprofessional conduct exhibited during different points throughout this investigation, you have left us with no choice but to advise our clients to assert their privileges in the face of these subpoenas," Levy wrote.

A former federal prosecutor, Renato Mariotti, said the First Amendment argument, while "novel," seemed "unlikely to succeed."

"That is probably why the attorneys have emphasized other arguments, like Nunes' apparent lack of authority to issue the subpoenas and the fact that Congress didn't authorize the investigation he's conducting on his own," Mariotti said.

"Those arguments have more merit, and they draw attention to Nunes' bizarre behavior over the past month — including his subpoenas to the DOJ," he added. "That said, if the subpoenas issued by Nunes are valid, then I expect these individuals will be held in contempt if they refuse to testify as ordered."

Republican Rep. Mike Conaway, who is now heading the committee's investigation, reportedly approved Nunes' subpoenas to Fusion GPS, but House Intelligence Democrats have indicated that they were not consulted.

Democratic Rep. Eric Swalwell, a member of the House Intelligence Committee, told Business Insider last week that "the only person who can intervene" to preserve the credibility of the committee's Russia investigation amid Nunes' recent antics was House Speaker Paul Ryan.

"If Speaker Ryan wants a credible investigation to come out of the House Intelligence Committee, he'll do everything he can to make sure Devin Nunes' fingerprints are not on our report," Swalwell said in an interview.

The Senate Judiciary Committee interviewed Simpson behind closed doors over the summer for roughly 10 hours. The committee is debating whether to release the transcript from the interview. Fusion GPS has been cooperating with the Senate Intelligence Committee since July, Levy wrote.

Read the full letter below: