Fusion GPS' Peter Fritsch and Thomas Catán invoked their Fifth Amendment rights not to answer questions during their closed-door appearance before the committee, according to their attorney Joshua Levy.
After the session, which lasted more than an hour, Levy charged that the committee broke with its past practices by requiring them to physically appear to plead the Fifth.
"No American should have to experience the indignity that occurred today," Levy told reporters. "No American should be required to appear before a congressional committee just to invoke his constitutional privileges. But that is what Chairman (Devin) Nunes required of our clients at Fusion GPS today, in a sharp departure from even the past practice of this committee's investigation, where witnesses under the exact same circumstances were excused from appearing."
On Monday, Fusion GPS responded to the committee's subpoena by accusing Nunes -- the California Republican who has stepped aside from the committee's Russia probe -- of a "clear abuse of power," arguing that the subpoena violated the partners' First Amendment rights.
Florida Rep. Tom Rooney, a senior Republican on the committee, defended the decision to hold a hearing for the Fusion GPS partners.
"We didn't require them to come in and plead the Fifth. We asked them to come in to answer questions with regard to our investigation, as we have with numerous, numerous witnesses over the last several months," Rooney said after the session concluded.
"The fact that they did plead the Fifth obviously is their right, but I think it is important that we hear from them directly, especially with regard to their extremely lengthy response to our request and in part, partially, because their response was that the investigation and subpoena was illegitimate. I think by the very fact that they were here shows its legitimacy and I think that's important," Rooney added.
A Democratic committee aide disputed that characterization, saying that the committee's practice thus far in the Russia investigation has been "not to require the witness to appear before the Committee for the sole purpose" of invoking the Fifth Amendment.
"Today's session, in which the witnesses were required to appear and respond to multiple questions, needlessly deviated from this sound and longstanding practice," the aide said. "It was all the more unwarranted because Fusion showed a willingness to cooperate with the committee. We would still like the opportunity to interview Fusion witnesses and this committee action has now made that needlessly difficult."
Nunes issued the subpoenas for the three Fusion GPS partners last week, which was approved by the Republican leading the Russia probe, Rep. Mike Conaway of Texas, but not the panel's top Democrat, Rep. Adam Schiff of California.
The subpoenas to Fusion GPS are only the latest issue stoking partisan tensions on the House panel's Russia probe, as well as Schiff's recent op-ed that some witnesses were being "rushed" before the committee, which sparked stiff Republican pushback.
The subpoenas issued to the Fusion GPS partners set a Wednesday deadline for Fritsch and Catan to respond. The committee is also seeking a November 8 deposition from Glenn Simpson, the firm's founder, who testified earlier this year in closed session before the Senate judiciary committee.
Nunes did not attend Wednesday's hearing, Levy said.
Spokespeople for Schiff, Nunes and Conaway did not immediately respond to requests for comment.
This story has been updated to include additional developments.