Mick Mulvaney, the acting White House chief of staff, said late Monday he planned to file a lawsuit addressing whether top Trump administration officials can be compelled to appear before House Democrats as part of the impeachment inquiry.

Mulvaney, represented by William Pittard of Washington’s KaiserDillon, had previously sought to join a case brought by former national security adviser John Bolton’s deputy, Charles Kupperman, who went to court seeking clarity on whether to comply with a House subpoena or follow White House instructions to not cooperate with the impeachment probe.