Justice Department lawyers under the Trump administration continued to fight the subpoena on other legal grounds, however, arguing that Congress had no “cause of action” that authorized it to sue the executive branch. (The executive branch has taken that position under administrations of both parties, and the Justice Department said it was “prepared to proceed” with the argument as scheduled if the court denied its request for a delay.)
The dispute is further complicated by the fact that there are so many participants — House Democrats, Mr. McGahn, the Biden administration and potentially Mr. Trump. The former president has not been a party to the lawsuit, but he might try to intervene and assert executive privilege — yet another issue that has not yet been adjudicated in the matter — if the executive branch under Mr. Biden drops out of the case.
Patrick F. Philbin, a former deputy White House counsel who is one of the people Mr. Trump designated to deal with residual issues related to presidential records, declined to comment.
William A. Burck, a lawyer for Mr. McGahn, has previously said that his client intended to defer to the president’s instructions, pending a final judicial order. A person familiar with the deliberations said Mr. Burck had not taken a position on what Mr. McGahn would do if Mr. Biden were to instruct him to talk to Congress, but Mr. Trump still told him not to.
Stuart F. Delery, a deputy White House counsel, said in an interview that the negotiations are still preliminary but that the Biden administration would like more time to try to resolve the dispute while preserving the “institutional interests connected to the presidency.”
There are few legal precedents. A rare and limited guidepost is a 1977 case, Nixon v. General Services Administration. In it, the Supreme Court ruled that Richard M. Nixon could assert executive privilege claims over official records from his White House even though he was no longer the president — but it also weighed that assertion against the contrary view of the sitting president at the time, Jimmy Carter.
That dispute, however, centered on control of Nixon-era White House documents, not a subpoena for a former lawyer’s testimony. Another question is how attorney-client privilege works for a former White House lawyer when the presidency changes hands — and what would happen if Mr. Trump were to file a bar ethics complaint going after Mr. McGahn’s law license if he cooperates with the House at Mr. Biden’s request but over Mr. Trump’s objections.