The House Judiciary Committee will subpoena former White House staff secretary Rob Porter as part of its probe into President Trump’s alleged attempts to obstruct special counsel Robert Mueller’s investigation, Politico reported Monday.

Porter, who resigned last year amid allegations of domestic abuse, served as a close confidante to the president during the Mueller probe and, as such, might be a valuable witness for House Democrats seeking to expose Trump’s misconduct as they weigh whether to begin impeachment proceedings.

The White House has thus far blocked senior White House officials from complying with lawmakers’ oversight requests, asserting that high-level officials enjoy “absolute immunity” from testifying before Congress. That claim is now being challenged in the courts by Democrats seeking testimony from former White House counsel Don McGahn.

Porter is mentioned multiple times in the Mueller report. In one section that will likely be of particular interest to lawmakers, the report details how Trump asked Porter to speak with senior Department of Justice official Rachel Brand about supervising the Mueller probe as a corrective to attorney general Jeff Sessions’s recusal.

The former White House aide never carried out the president’s request “because he was sensitive to the implications of that action and did not want to be involved in a chain of events associated with an effort to end the investigation or fire the special counsel,” according to the report.

Porter was also present for, and took contemporaneous notes related to, Trump’s repeated diatribes against McGahn. According to the Mueller report, some of those rants included threats to fire the White House lawyer over his adherence to a policy of cooperation of with the special counsel’s office, and his refusal to author a dishonest letter falsely denying a New York Times report that indicated the president ordered him to fire Mueller.