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.

The House Judiciary panel argues in Thursday’s brief that judges should be able to step in when executive officials defy congressional subpoenas, as they routinely do to enforce subpoenas in other types of disputes.

A ruling that kept Congress out of court would upend separation-of-powers principles and “encourage the Executive to defy Congressional subpoenas and refuse accommodation,” the brief states.

Congress can’t effectively legislate, conduct oversight or consider impeachment without information, including about presidential administration misconduct, the brief reads.

“If a subpoena is not judicially enforceable there is no reason to believe it would yield better results than a request,” the committee argues.

The panel also criticizes the previous ruling of a three-judge panel of the D.C. Circuit that found the judicial branch had no role in political disputes between the other two branches. The full D.C. Circuit vacated that panel ruling last month.

The panel decision to deprive Congress of the ability to enforce subpoenas in court against executive branch officials did not stay out of politics, but essentially sided with the executive, the committee argued.

“The panel suggested that Congress could use political tools —such as contempt and even impeachment — to force compliance, but the constitutional brinksmanship envisioned by the panel would heighten interbranch conflict and undermine, not protect, the separation of powers,” the committee brief states.

Government shutdowns arising from withholding appropriations shouldn’t be necessary to enforce supboenas, House Judiciary argues, and using the inherent contempt power to arrest recalcitrant witnesses and risk a physical confrontation also shouldn’t be necessary before the courts consider the case.

“The Constitution does not require Congress to resort to radical self-help,” the committee wrote.

The committee also points out that Trump’s lawyers defended him against obstruction-of-Congress charges in the Senate impeachment trial by arguing that the House should have first sought judicial enforcement of its subpoenas.

“In any event, recent experience confirms that political tools cannot force an obstinate Executive to cooperate with legitimate Congressional inquiries,” the brief states.