Acting White House chief of staff Mick Mulvaney filed a notice of withdrawal of his motion to join an ongoing lawsuit filed last month.
Kupperman was subpoenaed by House Democrats on Oct. 25, that subpoena has since been withdrawn. The House asked the judge to dismiss Kupperman's case on Wednesday after they withdrew their subpoena for Kupperman's testimony, arguing that the case was obsolete without the subpoena.
The president's former deputy national security adviser shares the same attorney with his former boss, ex-national security adviser John Bolton, who has rejected a House invitation for testimony. He has not be subpoenaed at this point in time.
In their filing on Monday, Democrats argued "this case is moot … (and) the subpoena to Kupperman, which provides the sole basis for the injury he asserts in his complaint, has been withdrawn." Therefore, Mulvaney should not be allowed to intervene. Democrats added that Mulvaney's intervention should be thrown out even if the "case were not moot." Democrats claimed that "Mulvaney is differently situated from Kupperman in several important respects that make clear that he lacks a cognizable interest in Kupperman's case."
On the other hand, Kupperman's attorney, Charles Cooper, also argued Mulvaney should not be able to join the suit but for different reasoning. Cooper in his filing argued that Kupperman is neutral, and noted that Mulvaney is not, writing in part, "Mulvaney has made it clear that he supports the Executive, and he accordingly seeks declaratory relief against only the House Defendants."
Attorneys also pointed out that Kupperman is no longer part of the administration, unlike Mulvaney.
Mulvaney's notice on Tuesday evening states his intention to refile as a separate, related case.
Kupperman filed his case shortly after he was subpoenaed, asking a judge to decide whether he must submit to questioning in the House impeachment inquiry.
The case is the latest twist in White House efforts to block all former and current administration officials from testifying or otherwise cooperating with the Democrat-led probe.
The White House had maintained that Kupperman is entitled to what it calls constitutional immunity, arguing, "Congress may not constitutionally compel the president's senior advisers to testify about their official duties" due to the separation of powers between the executive and legislative branches.