“I’m not inclined to grant the motion,” Leon said during an unusual, holiday telephone hearing Monday, according to a transcript of the 35-minute session released Monday night.

Less than an hour after the hearing wrapped up, Mulvaney’s lawyers submitted a short notice to the court saying they planned to drop their bid to join the pending case.

Mulvaney’s attorneys said they would instead file their own lawsuit seeking a ruling on whether the top Trump aide must comply with the subpoena issued last week or with instructions from Trump to ignore the demand for testimony.

Mulvaney’s lawyers, William Pittard and Christopher Muha, said they planned to designate the case as related to the suit Kupperman filed last month. That will cause Mulvaney’s case to be routed to Leon, an appointee of President George W. Bush.

However, the House or the Justice Department could object to the case being assigned to Leon, prompting more legal wrangling.

Kupperman brought his suit last month in what his counsel billed as an attempt to get a definitive ruling from a judge on whether a former White House aide must comply with a congressional subpoena over the objections of Trump, who has asserted “absolute immunity” to prevent his aides from testifying.

That lawsuit appeared to be moot after the House last week withdrew its subpoena for Kupperman, but Mulvaney’s unexpected request on Friday night to join the case suggested that it could get new life.

On Monday evening, Leon scheduled a hearing for Tuesday afternoon to which he’s summoned all the lawyers, including Mulvaney’s, to his courtroom to discuss the status of the case and whether there’s any objection to the acting White House chief of staff’s plan for a related lawsuit. Leon said he’d take up any proposed changes to the briefing schedule in the case but remained firm that “under no circumstances will the December 10, 2019 oral argument date be delayed.”

The rapid-fire filings appear to underscore the tension building among current and former Trump aides whose testimony could prove pivotal to the Democrats’ impeachment investigation.

Former national security adviser John Bolton, who is represented by the same attorney as Kupperman, has signaled a willingness to testify to House lawmakers but only if Kupperman loses in his court case. His lawyer last week said in a letter to House attorneys that Bolton had knowledge of “many relevant meetings and conversations” related to the impeachment inquiry.

In Monday’s filing to Leon, Kupperman’s attorney proposed that Mulvaney be allowed to pursue his complaint on a separate path that should be allowed to stay before the same judge because the cases are “related.”

But Kupperman’s attorney Charles Cooper said the two cases should be split up because they deal with different sets of facts and arguments. For one, Kupperman is neutral on whether Leon rules in favor of Congress or the president when it comes to “the merits of the unsettled Constitutional dispute at issue in this case.”

That’s in contrast to Mulvaney, who Cooper said “has made it clear that he supports the Executive, and he accordingly seeks declaratory relief against only the House Defendants.”

Cooper also told Leon that the cases should be separate because of which lawyers are making the arguments. While Mulvaney has a personal attorney representing him, the chief of staff’s interests are also “adequately protected by an existing party” in Trump and through the president via the Justice Department.

The department, Kupperman explained, will “vigorously champion Mulvaney’s interest in the President’s assertion of absolute immunity overriding Mulvaney’s obligation to appear and testify in response to the House subpoena.”

Another key difference between Mulvaney and Kupperman, Cooper explained, involves Mulvaney’s public comments last month that appeared to admit a quid pro quo relationship connecting Trump’s decision to hold back on releasing foreign aid to Ukraine until it agreed to an investigation into the president’s political opponents.

“An admission he subsequently sought to disavow,” Cooper wrote. But that contrasts with Kupperman, who Cooper said “has never publicly disclosed information relating to any of his official duties, including the matters under investigation by the House.”

“Accordingly, there is a serious question as to whether Mulvaney waived the absolute testimonial immunity claimed by the President such that a judgment in Plaintiff’s case upholding the claim of immunity will not necessarily apply to Mulvaney,” Cooper said.

During the hearing, Mulvaney’s attorney disputed that his client’s public statements waived any immunity.

“The idea that Mr. Mulvaney has somehow waived broad immunity by speaking about this at a press conference, I think, A, it doesn’t have any legs, and, B, it’s not the kind of issue that’s going to delay this case,” Muha said.

There are other reasons Mulvaney and Kupperman shouldn’t be on the same case dealing with the legal complexities associated with their titles, Cooper said.

While Kupperman is now a private citizen, any ruling on Mulvaney would broach issues involving a Justice Department argument that presidential advisers are immune from testifying because preparations would divert them from their regular duties. That’s not an issue in Kupperman’s case.

Kupperman’s lawyer also told Leon that his client might be immune from giving forced testimony because all of his work involved matters of national security and foreign affairs. By contrast, Cooper said, “the bulk” of Mulvaney’s work doesn’t deal with those topics.

Also involved in the dispute are House Democratic lawyers who have been trying to get testimony from both Kupperman and Mulvaney.

Doug Letter, the chief counsel for the House, said in a brief that Mulvaney’s effort to intervene should be treated with suspicion because the testimony the impeachment investigators have collected to date places him at the center of alleged wrongdoing against Trump.

Unlike Bolton’s former deputy, Mulvaney does not say he’s willing to comply with the House subpoena even if a court rejects his claim of “absolute immunity” from testifying.

More fundamentally, House lawyers say Mulvaney filed his intervention too late: The House has already withdrawn its subpoena for Kupperman, rendering the initial case defunct, House lawyers say.

“The House has no intention of reissuing compulsion to him; therefore, it’s our position that it’s impossible for the Court to grant any effectual relief here, and, thus, the underlying case is moot,” House Deputy General Counsel Todd Tatelman told Leon during the phone hearing.

“Just about every court that has addressed that question appears to have held that intervention is not permitted to breathe life into a non-existent suit,” Tatelman added.

Finally, the House argued that Mulvaney — given his closeness to Trump — probably had a hand in the White House’s decision to declare himself immune from responding to a congressional subpoena.

“Allowing him to intervene to address that would seem incredibly inappropriate under those circumstances,” Tatelman said.

House lawyers did suggest they’re open to receiving a friend-of-the-court brief from Mulvaney about his view of the matter.

Attorneys for several news outlets, including POLITICO, formally requested real-time access to Monday’s telephone hearing, but the judge rebuffed them. He said that the matter was “time-sensitive” and that the courthouse was closed for the Veterans’ Day holiday.

In a brief order minutes before the hearing, Leon said a transcript of the telephone hearing would be made public. It was released Monday night, about three hours after the court session ended.