The federal judge overseeing former President Donald J. Trump’s classified documents case said on Thursday that she intended to look anew at a hugely consequential legal victory that prosecutors won last year and that served as a cornerstone of the obstruction charges filed against Mr. Trump.

In her ruling, the judge, Aileen M. Cannon, said she would hold a hearing to reconsider another judge’s decision to allow prosecutors to pierce the attorney-client privilege of one of Mr. Trump’s lawyers under what is known as the crime-fraud exception.

That provision allows the government to get around the normal protections afforded to a lawyer’s communications with a client if it can prove that legal advice was used to commit a crime.

Depending on how Judge Cannon ultimately rules, her decision to redo the fraught and lengthy legal arguments about the crime-fraud exception could deal a serious blow to the obstruction charges in the indictment of Mr. Trump. Even if she ends up confirming the initial judge’s findings, holding yet another hearing on the issue will take more time and play into Mr. Trump’s strategy of delaying the case from going to trial for as long as possible.

Judge Cannon’s decision, contained in an 11-page ruling, came two days after Mr. Trump’s lawyers and prosecutors in the office of the special counsel, Jack Smith, held a sealed hearing in Federal District Court in Fort Pierce, Fla., to discuss whether to relitigate the battle over the crime-fraud exception.

At the hearing, according to the ruling, Mr. Smith’s deputies told her that a new proceeding on the question would “devolve into a ‘mini-trial’” that might subject some of their witnesses to cross-examination before the actual trial began.

Judge Cannon, however, had little patience for that argument, noting there was a difference between “a resource-wasting and delay-producing ‘mini-trial’” and an evidentiary hearing “geared to adjudicating the contested factual and legal issues.”

In a prickly manner that she has shown before, Judge Cannon then asserted her authority to conduct the hearing.

“It is before this court — in this post-indictment context — to make factual findings on contested questions pertinent to the second prong of the crime-fraud exception,” she wrote, using italics to emphasize her point.

The battle over the crime-fraud provision, which unfolded in early 2023 in Federal District Court in Washington, where the investigation into Mr. Trump began, was one of Mr. Smith’s most significant legal successes.

It was capped by an expansive ruling by Judge Beryl A. Howell, who was serving at the time as Washington’s chief federal judge, that determined that legal work by one of Mr. Trump’s lawyers, M. Evan Corcoran, had most likely been used in the commission of a crime.

Because of that finding, Judge Howell permitted prosecutors to question Mr. Corcoran in front of a grand jury and compelled him to turn over two extremely revealing recordings he had made of himself describing his interactions with Mr. Trump in an early phase of the documents inquiry.

Some of what the government learned from Mr. Corcoran later showed up in the indictment against Mr. Trump. It was used to bolster damning accusations that the former president had repeatedly sought to obstruct attempts by the Justice Department to retrieve a trove of highly classified state secrets he had kept after leaving the White House.

Mr. Corcoran told investigators, for example, that Mr. Trump had asked him what would happen if he refused to “play ball” with prosecutors who had obtained a grand jury subpoena for all of the classified documents in his possession.

Moreover, Mr. Corcoran’s recordings described how Mr. Trump made a “plucking motion” after he had placed about 40 secret documents in a folder in preparation for handing them over to the prosecutors in compliance with the subpoena.

Mr. Corcoran said the gesture made him think that Mr. Trump was suggesting that he should take the folder to his “hotel room and if there’s anything really bad in there, like, you know, pluck it out.”

Throughout the classified documents case, Judge Cannon, who was appointed by Mr. Trump, has often shown a tendency for wanting to decide legal questions on her own even if the issues under consideration have a long history of having already been settled.

Last Friday, for instance, she held a hearing on whether Mr. Smith had been constitutionally appointed as special counsel despite the fact that courts reaching back to the Watergate era have repeatedly upheld the viability of independent prosecutors. She has not yet issued a ruling on that question, which she could have decided on the merits of written filings alone.

As part of her ruling on Thursday, Judge Cannon denied a request by Mr. Trump’s lawyers to hold a separate hearing to determine if federal investigators had lied in order to obtain a warrant to search Mar-a-Lago, his private club and residence in Florida. During the search, which took place in August 2022, F.B.I. agents discovered more than 100 classified documents, some of which are now at the heart of the case.

But Judge Cannon said Mr. Trump’s attempts to challenge the legality of the Mar-a-Lago search could nonetheless continue — albeit on much narrower legal grounds.

The judge said she wanted to examine further whether the agents who searched Mr. Trump’s estate had been given proper guidance in the warrant about the types of documents they were authorized to seize. While she acknowledged it was clear that the agents had permission to take any documents with classification markings, she wanted to know more, she said, about what they were told about phrases in the warrant, like “presidential records” and “national defense information.”

Those terms, which were used to describe the sorts of documents that could be hauled away, “do not carry ‘generally understood meanings,’” she wrote, “such that a law enforcement agent, without further clarification, would have known to identify such material as ‘seizable’ property.”

Her remedy for determining whether the agents knew enough about the phrases?

Yet another hearing, which she plans to schedule soon.