The House’s lawyers cited the recent missive from the current top White House lawyer declaring that the Trump administration would refuse all cooperation with the House’s impeachment inquiry, calling it “illegitimate” and “invalid.” The blanket stonewalling, House lawyers say, requires a court to intervene or else Trump could be effectively shielded from accountability.

“If the president could deprive the committee of information required for its impeachment inquiry into his own misconduct, the president could potentially thwart his accountability for that conduct,” the attorneys said, referring to Trump’s efforts to block witness testimony and document production.

“No further discussion will resolve an impasse dictated by the president himself,” they added.

McGahn was subpoenaed earlier this year to testify about Trump’s efforts to undermine or even shut down the Mueller investigation, but the White House directed him not to comply, asserting that former senior advisers to the president have “absolute immunity” from testifying before Congress.

Since then, House Democrats have rapidly intensified their impeachment inquiry, which now focuses on Trump’s efforts to pressure Ukraine’s leaders to investigate his political rivals. White House Counsel Pat Cipollone’s letter to Speaker Nancy Pelosi last week slamming the impeachment inquiry as illegitimate and invalid effectively shut down any possibility of cooperation.

The House lawyers emphasized why they consider McGahn — more than any other witness — the most important to determine whether Trump should be impeached on the basis of obstructing the Mueller investigation.

McGahn, they argued, could speak to Trump’s “demeanor, state of mind, and knowledge of wrongdoing during their interactions.” He was a witness to numerous allegations of potential obstruction, from Trump’s direct requests to remove Mueller to his efforts to pressure then-Attorney General Jeff Sessions to constrain the special counsel’s investigation.

“McGahn’s testimony on these topics will help the committee determine all of the relevant facts, present a full picture of what happened, and decide whether to recommend articles of impeachment,” the lawyers added.

McGahn’s testimony is even more urgent, they said, because Trump has repeatedly tried to discredit him, claiming that he lied to Mueller’s team about the episodes he witnessed.

The White House’s efforts to undercut House Democrats’ investigations intensified last month when Pelosi announced a formal impeachment inquiry centered on Trump’s interactions with the Ukrainian president. Since then, the Trump administration has sought to block current and former senior officials and diplomats from testifying — though some have appeared for depositions regardless of those directives.

But executive branch agencies are still refusing to comply with subpoenas for documents. This week alone, the Pentagon, the White House budget office and the vice president’s office all rejected subpoenas or requests for documents related to Ukraine.