The House Judiciary Committee warned a federal appeals court Thursday that the wrong decision in a separation-of-powers showdown with the Trump administration over the enforcement of subpoenas could “effectively eliminate Congressional oversight as we know it.”

Committee lawyers filed the brief as part of the House effort to force former White House counsel Don McGahn to testify about events detailed in the Mueller report. The Trump administration has argued the close presidential adviser enjoys “absolute immunity” from such testimony.

The full U.S. Court of Appeals for the District of Columbia Circuit is set to hear oral arguments April 28 on the committee’s McGahn subpoena, as well as the House’s effort to stop the construction of a wall on the U.S. border with Mexico.

In both lawsuits, the full D.C. Circuit will focus on whether the House has the legal right to bring each of the suits in the first place.

Some House Democrats point to the McGahn case as the test case for enforcing more subpoenas in their oversight investigations into the Trump administration, which culminated in December with the House impeachment of President Donald Trump for abuse of power and obstruction of Congress.