More significantly, Jackson’s ruling could also fuel Democrats’ investigation into Trump’s posture toward Ukraine, which has already brought the president to the brink of being impeached.

Trump has indicated that at least three senior aides sought by Democrats — former national security adviser John Bolton, Bolton’s deputy, Charles Kupperman, and acting chief of staff Mick Mulvaney — are “absolutely immune” from testifying in the impeachment probe.

In her ruling, Jackson emphasizes that arguments that former White House aides enjoy “immunity” from congressional testimony are even more spurious.

“[I]t would seem that if one’s access to the Oval Office is the reason that a categorical exemption from compelled congressional process is warranted,” she writes, “then that trump card should, at most, be a raincheck, and not the lifetime pass that DOJ proposes.”

This aspect of Jackson’s ruling could become relevant as Kupperman has sought legal clarity about whether he is permitted to testify or must defer to the president’s “absolute immunity” claim. Jackson answers the question flatly.

It’s not clear, though, that even this favorable ruling will slow Democrats’ march toward impeachment as they work to scoop up more evidence. In a letter to colleagues Monday from House Intelligence Chairman Adam Schiff, the leader of the Ukraine probe, the California Democrat wrote that he’s unwilling to let ongoing court battles sideline the Democrats’ “overwhelming” case against Trump.

But the House has also signaled it is ready to take advantage of any legal wins it can chalk up in the meantime.

“I am pleased the court has recognized that the Trump Administration has no grounds to withhold critical witness testimony from the House during its impeachment inquiry,” said Rep. Jerrold Nadler (D-N.Y.), the Judiciary Committee chairman.

The chamber’s lead counsel, Doug Letter, wrote in a recent court filing that the Judiciary Committee could arrange for McGahn’s testimony next month while also beginning to consider articles of impeachment.

One potential limitation: Jackson underscored that her ruling was silent on executive privilege, which means McGahn and any other witness who comes before Congress may still decline to answer questions but must do so on a case-by-case basis. Jackson said she set aside questions of privilege under an agreement with the House and the Justice Department to resolve the immunity question more quickly.

“No one contests … that a senior-level aide in [McGahn’s] position has no right to invoke executive privilege to withhold certain information in the course of his testimony, as appropriate,” Jackson notes.

Multiple witnesses in the House’s Ukraine probe described Bolton as opposing Trump’s decision to withhold a $400 million package of military aid to Ukraine. The move came as Trump and his personal lawyer Rudy Giuliani were pressuring the country’s new president to investigate Trump’s political adversaries.

Former national security adviser John Bolton. | Win McNamee/Getty Images

Mulvaney acknowledged at an Oct. 17 press conference that the aid freeze was connected to Trump’s demand for investigations, but he later retracted his comments and has said little on the subject since. Other witnesses have described Mulvaney as a central player in what they say was a quid pro quo, in which Trump conditioned the military aid and a White House visit on Ukraine opening up the politically advantageous probes.

The House subpoenaed Kupperman but later withdrew their demand after he sued and sought a judge’s ruling on the matter. Democrats accused Kupperman, and by extension Bolton, of a delay tactic since court proceedings could take months. They instead urged the two men to honor Jackson’s ruling in the McGahn matter since it has always been expected this month.

Kupperman and Bolton haven’t signaled whether they’ll voluntarily comply with the McGahn result. The duo’s attorney, Charles Cooper, has noted significant differences between their cases and the Democrats’ demands to hear from McGahn. That’s because a judge in the one previous court case that dealt with “absolute immunity” determined that “national security” could be a valid reason to invoke the protection.

While McGahn’s purview at the White House was broad and touched on many subjects, Bolton and Kupperman dealt in their work almost exclusively on matters that would fall into that protected category, Cooper said.

Mulvaney also tried to join Kupperman’s suit but he quickly backed down and later opted against filing his own litigation.

The Mueller report in recent weeks has taken a backseat to the Ukraine investigation, though Letter’s court filing last week said the Judiciary Committee was eyeing an impeachment hearing next month that would again seek to include McGahn’s testimony.

At the same time, House Speaker Nancy Pelosi and other top Democrats have insisted they won’t hinge their impeachment strategy on the outcome of pending judicial rulings.

“We cannot be at the mercy of the courts,” Pelosi told reporters last week. “The courts are very important in all of this. Those cases will continue. But I have never said we cannot proceed without the courts. Because that's a technique on the part of the administration: just keep ratcheting up to a higher court.”

McGahn sat for five voluntary interviews with Mueller staffers starting in late 2017, and his recurring role in the special counsel’s final report made him a prime witness for House investigators before the Ukraine scandal broke. Democrats in April issued a subpoena for documents and testimony from McGahn, but he skipped out on a scheduled hearing appearance with backing from a new DOJ legal opinion.

White House and Democratic lawyers negotiated through the summer but failed to reach a deal to get McGahn’s testimony, prompting the House’s lawsuit in August.