A federal judge in Boston ruled for the third time on Friday that the Trump administration’s plans to restrict mail-in voting are likely illegal and it should be prevented from implementing them ahead of the midterm election.

Judge Indira Talwani had already temporarily blocked the U.S. Postal Service from moving forward, in a ruling that was set to expire Sept. 10. The new order extends that block indefinitely.

What the ruling did not do was move the issue much closer to a resolution, even with time rapidly running out for state election officials to start distributing mail-in ballots to voters. The Trump administration had already appealed her temporary block to the Supreme Court, and the justices are likely to have the final word on President Trump’s plans ahead of the election.

The ruling came just a day after Judge Talwani held a two-hour hearing in Boston, with lawyers representing large coalitions of Democratic-led states and voting rights groups squared off with lawyers representing the Postal Service and a smaller group of Republican-led states that support the rule.

Judge Talwani reviewed their concerns about the Postal Service rule, which would require states to report to the Postal Service which of their voters are qualified to use mail-in ballots, and to adopt a new ballot envelope designed to be scanned and linked to voters. The Postal Service could withhold mail delivery of ballots in states that don’t comply.

She said she believed the concerns among states and election officials that it was too late to adopt the changes without widespread mishaps and delays. The election is now only 60 days away.

Judge Talwani wrote the rule appeared to force states to work against their own privacy laws and imposed requirements that appeared certain to result in some voters being disqualified.

“Having considered this series of hurdles for officials against the backdrop of an impending election, and mindful of the fact that any minor malfunction will result in the denial of mail ballots, the court concludes that disenfranchisement for plaintiff organizations’ members, regardless of residence, is practically inevitable,” Judge Talwani wrote.

In particular, Judge Talwani questioned why the Postal Service was moving ahead with exceptional urgency to get the rule approved in time for the midterms while also insisting it was a relatively routine action that required only minor clerical changes. She cast doubt on Mr. Trump’s claims that the rule was necessary for election security, writing that the Postal Service’s rationale “includes no evidence relating to fraudulent mail voting to support the rushed implementation.”

Still, Judge Talwani wrote that she would not second guess or seek to prevent any of a number of Republican-led states that support the rule from meeting its requirements voluntarily. She also said the decision on Friday addressed only the implementation of the rule for the current election cycle, and that courts could later sort out whether it was lawful in its entirety.

Like in her two previous rulings, she wrote that the rule appeared to exceed the Postal Service’s statutory authority by assigning broad authority to the service to regulate election mail, beyond what lawmakers intended. She added that with each passing week, it had become increasingly unrealistic to expect states to be able to comply.

Jeff Jackson, the North Carolina attorney general, said in a statement that the administration’s request to the Supreme Court on Thursday came just as the state was scheduled to send out ballots to voters on Friday.

“That would throw our election into chaos,” Mr. Jackson said. “The ballots are printed, the process is underway, and North Carolinians deserve a lawful and orderly election.”