Acting White House Chief of Staff Mick Mulvaney moved Friday night to join a pending lawsuit asking a judge if he must comply with a House subpoena, the hearing for which he’s already skipped.

The Friday night filing, first reported by Politico, may re-up the relevance of the suit filed last month by former deputy to ex-NSA John Bolton Charles Kupperman, who similarly sought advice as to whether or not his testimony was protected by immunity, as the White House claims.

Kupperman’s suit is still pending despite the fact that the House committees tossed his subpoena rather than fight him in court.

“The question whether the President’s authority must give way in the face of a congressional subpoena—the determination Mr. Kupperman has asked this Court to make—is central to the question whether the House may take adverse action against Mr. Mulvaney, as threatened. For that reason, Mr.Mulvaney seeks to intervene here,” wrote Mulvaney’s lawyers.

So far, Democrats have opted to brush off witnesses resistant to testifying, choosing to focus on officials willing to speak out.