Washington CNN  — 

On Monday, a federal judge plans to make the first major court ruling in the fight between the House and the White House over impeachment witnesses.

Judge Ketanji Brown Jackson says she will decide by the close of business whether former White House counsel Don McGahn must testify about President Donald Trump to Congress.

Until this point, the case has lingered in the background, stemming from a subpoena the House Judiciary Committee sent to McGahn in April, well before the Ukraine impeachment scandal kicked House proceedings into high gear this fall. But depending on Jackson’s ruling, McGahn’s case could be pivotal for the House as it considers impeaching the President on multiple counts.

A ruling in the House’s favor, for instance, could encourage resistant witnesses from the administration to testify and could bolster any case the House makes to impeach the President for obstructing its impeachment efforts.

The House sued McGahn in August, claiming the Judiciary Committee needed his public testimony about Trump’s attempts to obstruct the Russia investigation. McGahn had been a key witness on several instances of obstruction that special counsel Robert Mueller investigated, and had since left the White House.

But McGahn never appeared before Congress. The White House had claimed he had “absolute immunity” and could ignore the call for him to testify.

The White House has since used the same idea to block several more witnesses in the House impeachment probe, including top former national security officials with knowledge of the President’s dealings with Ukraine.

All along, the House has called that assertion unlawful and has waited for a judge to weigh in.

Judge’s roles

McGahn’s case headed to the judge in DC District Court near the height of the House’s Ukraine impeachment furor.

On Halloween, Jackson heard arguments from House lawyers and the Justice Department, which defends the White House and McGahn.

The judge, during the hearing, seemed floored that the White House was attempting to control even its former officials’ public statements.

“We don’t live in a world where your status as a former executive branch official somehow shields you or prevents you from giving information,” Jackson said. “I see almost every day people who are former executive branch officials giving information to the media.”

That same afternoon, another federal judge began considering the case of a different White House witness, former deputy national security adviser Charles Kupperman, whom the White House blocked from speaking about Ukraine, claiming the same type of “absolute immunity.”

In that case, the judge is about a month away from being able to rule, keeping Kupperman and his former boss John Bolton on ice.

The House has argued that the coming decision in McGahn is all they should need as a guide – but the national security advisers’ lawyer has made clear they may not follow in line.

NEW YORK, NY - NOVEMBER 15: Don McGahn, general counsel for the Trump transition team, gets into an elevator in the lobby at Trump Tower, November 15, 2016 in New York City. President-elect Donald Trump is in the process of choosing his presidential cabinet as he transitions from a candidate to the president-elect. (Photo by Drew Angerer/Getty Images)
Democrats say this White House lawyer is key in impeachment
01:30 - Source: CNN

Potential outcomes

Even so, a ruling Monday in favor of the House could help make the case that other former or even current White House officials should testify.

If Jackson says McGahn must testify, he could be called to the House right away, needing further action from the courts to avoid him being held in contempt.

He and the coming ruling also could become a key part of other potential impeachment avenues. The House has already hinted it’s considering impeaching Trump on obstructing their proceedings, including by stopping witnesses with the “absolute immunity” claim, making a federal court’s judgments even more significant.

Any increased chance that McGahn and others should now testify would be a triumph for the House, enabling it to draw even more attention toward arguments against the President. The House said it needed a ruling quickly regarding McGahn because he could speak to them about Trump potentially obstructing justice, and even perhaps Trump lying in written answers to the special counsel.

McGahn is the “principal witness,” the House has told the court, on evidence that Trump obstructed justice. The Judiciary Committee says it wants him to testify after the House Intelligence Committee’s hearings on Ukraine end.

(The Ukraine inquiry could lead to separate impeachment charges of bribery or abuse of office.)

But if the House loses in the McGahn case Monday, the White House’s “absolute immunity” would stay strong for now, and one of the most significant witnesses against the President –and others – would be protected behind the firewall.