Nellie Ohr, who worked as a contractor for the firm behind the anti-Trump dossier and whose Justice Department husband became a backchannel for passing along that information, has refused multiple requests to appear voluntarily before House committees, Fox News has learned.

Former FBI general counsel James Baker also has refused similar requests.

The House judiciary and oversight committees have spoken to numerous other witnesses, including Nellie’s husband Bruce Ohr, as part of their joint investigation into actions taken by DOJ during the Hillary Clinton email and Russia investigations. Bruce Ohr testified in late August about his relationship with former British spy and dossier author Christopher Steele.

But with the committee unable to secure a commitment to appear from Nellie Ohr or Baker, a Republican House Judiciary Committee aide indicated subpoenas are an option.

“The Committee continues to seek the testimony of Ohr and Baker and will compel their testimony if necessary,” the aide said.

The Ohrs’ role has become a key focus for Republican congressional investigators. They first alleged in the January 2018 House Intelligence Committee FISA memo that Ohr was the backchannel for Steele after he was fired by the bureau in November 2016 over his contacts with the media.

Investigators want to hear specifically from Nellie Ohr, as she worked as a contractor for the opposition research firm Fusion GPS, which was behind the dossier, and did Russia research.

Bruce Ohr told the FBI about his wife’s work for Fusion GPS, as well as his reservations about the credibility of the document and Steele’s animus for then-candidate Donald Trump. However, this was not shared with the Foreign Intelligence Surveillance Court when the dossier was used to help secure a surveillance warrant for then-Trump campaign aide Carter Page.

Congressional Republicans argue that the dossier was improperly used to obtain that warrant, and subsequent renewals.

Amid the probe, Trump on Monday took the extraordinary step of ordering the declassification of key documents related to the FBI investigation of Russian meddling in the 2016 election. This includes 21 pages of an application for a renewed surveillance warrant against Page, and all FBI reports on interviews with Bruce Ohr.

James Baker, as former FBI general counsel, also is a focus of the Republican investigation into alleged FISA abuse, and congressional sources believe Baker can explain why information about Steele’s bias, among other issues, was withheld from the national security court.

Baker, who has been teaching at Harvard Law School, has not responded to a request for comment. Lawyers for Fusion GPS said they are not representing Nellie Ohr. A message left at a number connected to an Ohr home address was not immediately returned.

Democrats, meanwhile, accused Republicans of crossing the line in their dig for information.

House Intelligence Committee ranking member Adam Schiff, D-Calif., called Trump's declassification decision "a clear abuse of power."

"[Trump] has decided to intervene in a pending law enforcement investigation by ordering the selective release of materials he believes are helpful to his defense team and thinks will advance a false narrative," Schiff said. "With respect to some of these materials, I have been previously informed by the FBI and Justice Department that they would consider their release a red line that must not be crossed as they may compromise sources and methods.

"This is evidently of no consequence to a President who cares about nothing about the country and everything about his narrow self-interest," Schiff added.

Fox News’ Samuel Chamberlain contributed to this report.