WASHINGTON — The former White House counsel Donald F. McGahn II must testify before House impeachment investigators about President Trump’s efforts to obstruct the Mueller inquiry, a judge ruled on Monday, saying that senior presidential aides must comply with congressional subpoenas and calling the administration’s arguments to the contrary “fiction.”

The 120-page decision by Judge Ketanji Brown Jackson of the Federal District Court for the District of Columbia handed another lower-court victory to House Democrats in their fight to overcome Mr. Trump’s stonewalling. The Justice Department, which is representing Mr. McGahn in the lawsuit, will appeal, a spokeswoman said.

“Presidents are not kings,” Judge Jackson wrote, adding, “They do not have subjects, bound by loyalty or blood, whose destiny they are entitled to control.”

Still, the ruling could have broader consequences. In rejecting the Trump administration’s sweeping claim that top presidential advisers, as Mr. McGahn was, are absolutely immune from being compelled to talk about their official duties — meaning they do not even have to show up — the judge said the same is true even for those who worked on national security issues.

Notably, John R. Bolton, Mr. Trump’s former national security adviser, has let it be known that he has significant information about the Ukraine affair at the heart of the impeachment inquiry but is uncertain whether any congressional subpoena for his testimony would be constitutionally valid. He wants a judge to decide.

Judge Jackson’s ruling also came on the same day that another federal judge in Washington held out the possibility that more documents about the Ukraine affair could yet see the light of day, ruling that emails between the White House and the Pentagon about the freezing of military aid to Ukraine should be released under a Freedom of Information Act lawsuit.

But even as those rulings suggested that more potential evidence for impeachment investigators might become available as the cases play out, House Democrats said the Intelligence Committee would deliver a report soon after Thanksgiving making the case for impeaching Mr. Trump, moving forward rather than waiting for the inevitable appeals to drag on.

Democrats are compiling a list of “noncompliance with lawful subpoenas” as part of the report so the Judiciary Committee can consider drafting an article of impeachment charging Mr. Trump with obstructing Congress, the intelligence panel’s chairman, Representative Adam B. Schiff of California, wrote in a letter to colleagues on Monday.

Several potential witnesses to what Mr. Trump said and did to pressure Ukraine to announce investigations that could benefit him politically — like Mr. Bolton and Mr. Trump’s acting chief of staff, Mick Mulvaney — have declined to testify because the administration instructed them not to, claiming that current or former senior officials are constitutionally immune.

Mr. Bolton, who met alone with Mr. Trump about why he was freezing a military aid package to Ukraine in August, has threatened to sue if Democrats try to compel him to testify, seeking a court ruling about whether such a subpoena is legally valid.

A lawyer for Mr. Bolton, Charles J. Cooper, has previously argued that Mr. Bolton’s situation is different from Mr. McGahn’s because Mr. Bolton’s official duties centered on foreign affairs and national security matters. But Mr. Bolton’s intentions and desires are unclear.

Mr. Bolton has become an enigmatic figure in the impeachment drama. According to other testimony, he strongly opposed the Ukraine pressure campaign and told aides to report what was going on to White House lawyers. He left the White House under rancorous circumstances in September and has since criticized Mr. Trump’s foreign policy.

But it remains unclear what he would tell impeachment investigators if he were to appear, and House Democrats are nervous that he is such a wild card he could just as easily hurt their case as help it. He accused the White House last week of not giving him back his Twitter account when he left, then teasingly asked if it was “out of fear of what I may say?”

In her ruling, Judge Jackson appeared to respond to Mr. Cooper’s notion. She wrote that the law required not just Mr. McGahn, but also “other current and former senior-level White House officials” who receive a subpoena to appear — and that it made no difference if they worked on domestic or national security matters.

“It is indisputable that current and former employees of the White House work for the people of the United States, and that they take an oath to protect and defend the Constitution,” wrote Judge Jackson, who was appointed by President Barack Obama.

Still, she emphasized, her ruling is only about whether Mr. McGahn must show up to be asked questions. It leaves unanswered whether the questions that lawmakers want to ask him — primarily about conversations with Mr. Trump detailed in the Mueller report — are subject to executive privilege, suggesting that even if Congress ultimately wins a Supreme Court ruling forcing Mr. McGahn to show up, the litigation process might have to start all over again.

The House Judiciary Committee subpoenaed Mr. McGahn in May after the release of the report by the special counsel, Robert S. Mueller III. It showed that Mr. McGahn was a key witness to several of the most serious episodes in which Mr. Trump sought to obstruct the Russia investigation.

But Mr. Trump, who had openly vowed to stonewall “all” oversight subpoenas after Democrats took control of the House in the 2018 midterm election, instructed Mr. McGahn not to cooperate.

In August, the House Judiciary Committee sued Mr. McGahn, seeking a judicial order that he comply with the subpoena. That same day, the panel also asked a judge for an order permitting it to see secret grand jury evidence gathered by Mr. Mueller, which Attorney General William P. Barr declined to provide to Congress. (Another federal judge ruled for Congress in the grand jury case a month ago, but the administration has appealed.)

The court filings said the House needed the information not just for oversight purposes, but also for an impeachment inquiry. While the impeachment focus has since shifted to the Ukraine affair that burst into public view in September, House Democrats are still considering an article of impeachment that would accuse Mr. Trump of obstruction of justice.

A question pervading both disputes is whether the Constitution permits Congress to subpoena aides to a president like Mr. McGahn and, potentially, Mr. Bolton, to talk about their official duties — or whether the president’s secrecy powers make his aides absolutely immune from such subpoenas.

Administrations of both parties have taken the position that “Congress may not constitutionally compel the president’s senior advisers to testify about their official duties,” as a 15-page legal opinion from Steven A. Engel, the head of the Justice Department’s Office of Legal Counsel, put it. But there is no definitive court precedent on the issue.

In 2008, another Federal District Court judge, John D. Bates, rejected that theory in a subpoena dispute. He ruled that President George W. Bush’s former White House counsel Harriet Miers had no right to skip a hearing for which she had been subpoenaed. Judge Bates, a Bush appointee, said she had to show up — although she might still refuse to answer specific questions based on a claim of executive privilege.

But because the Miers dispute was then resolved before an appeals court weighed in, Judge Bates’s opinion does not count as a controlling precedent for other disputes raising the same issue. That left the Obama administration, in a 2014 memo, free to take the position that Judge Bates had been wrong, and the Trump legal team echoed that logic.

In declaring that absolute immunity from congressional subpoenas for senior-level presidential aides “simply does not exist,” Judge Jackson spoke scornfully of the memos by the Office of Legal Counsel, sometimes called O.L.C., saying otherwise.

“Absolute testimonial immunity for senior-level White House aides appears to be a fiction that has been fastidiously maintained over time through the force of sheer repetition in O.L.C. opinions, and through accommodations that have permitted its proponents to avoid having the proposition tested in the crucible of litigation,” she wrote.

Peter Baker and Nicholas Fandos contributed reporting.