A preliminary order Monday by the Supreme Court granting President Trump the authority to move ahead with aspects of his plan to restrict mail-in ballots is unlikely the final verdict on his efforts to overhaul the voting process.

But the emergency opinion did immediately help Mr. Trump in one clear way.

Whether the president ultimately wins or loses in the courts, the latest ruling bolstered his longstanding efforts to seed doubt about the voting process, voting rights advocates said, a campaign that Mr. Trump has ramped up ahead of midterm elections that could see significant gains for Democrats.

“The purpose is not for legal victory,” said Michael McNulty, the senior policy director for Issue One, a nonpartisan group that tracks election issues. “The purpose is exactly to sow chaos, to create fear and confusion and confuse voters and attempt to undermine confidence” in the outcome.

The president’s attempt to have the U.S. Postal Service determine whether voters can cast ballots by mail this fall will likely alarm and confuse the public as multiple court fights play out, according to state officials.

Tobias Read, Oregon’s Democratic secretary of state, noted that his state would in a matter of weeks be sending out ballots to overseas voters and members of the military.

“Now is not the time to make major changes to how elections are run in Oregon,” Mr. Read said at a news conference Tuesday, adding: “This election is effectively underway.”

In March, Mr. Trump signed an executive order that sought to restrict who could vote by mail. It called on the Department of Homeland Security to compile state-by-state citizen lists to help determine voter eligibility and send that information to the states, and instructed the Postal Service to verify which ballots could be mailed, based on voter roll lists provided by the states.

The order has been challenged through multiple lawsuits filed by Democratic state attorneys general and voting rights groups. They have argued that the move amounts to a federal intrusion into the voting process, which the Constitution says is the domain of the states. The plaintiffs have also said that the order could disenfranchise people who rely on voting by mail to participate in elections.

The administration’s attempt to seize control of mail voting is part of a sweeping effort by Mr. Trump in his second term to use the levers of the federal government to assert control over elections.

The president, who has voted by mail himself, has tried to restructure the leadership of the Postal Service by installing more politically aligned officials. He has enlisted the nation’s intelligence agencies, the F.B.I., the Justice Department and the Department of Homeland Security in the broader mission of pursuing his election grievances.

Critics say that his goal is to undermine the results of the November elections in case they result in Democratic congressional majorities.

Lauren Bis, a White House spokeswoman, rejected that assertion.

“Radical Democrats continue to oppose common-sense measures that protect the security of mail-in ballots and ensure only Americans are electing American leaders,” Ms. Bis said in a statement. “The Trump administration will continue to lawfully enact the agenda President Trump was elected on, which includes the safety and security of our elections.”

Many of Mr. Trump’s attempts to influence the voting process have faced legal setbacks. The administration has lost all 23 cases related to its efforts to obtain state voter rolls.

While the administration secured a rare win on Monday at the Supreme Court, the order was preliminary, and the justices cautioned that the decision should not be construed as a final determination on the legality of Mr. Trump’s mail ballot order.

A separate injunction currently prevents the Postal Service from carrying out parts of the president’s executive order. And the broader legal battle over the order may soon return to the justices.

Still, Justice Ketanji Brown Jackson warned in her dissent on Monday that the court’s order would have an immediate effect. The ruling, Justice Jackson said, “needlessly injects chaos and uncertainty into the upcoming midterm elections.”

Americans have become increasingly reliant on voting by mail: Nearly one in three voters used the method in the most recent midterm and presidential elections. But polling indicates that voter confidence in elections has diminished as Mr. Trump continues to push debunked conspiracy theories about voting.

The percentage of Americans confident that their state and local governments would run a fair and accurate election dropped earlier this year to its lowest since at least 2020, according to a PBS News-NPR-Marist poll.

Mr. Trump’s efforts to restrict mail voting have also frustrated members of his own party. Senator Susan Collins of Maine, who is facing a tough re-election race this year, last weekend criticized the administration’s attempts to restrict the practice.

“Mail-in ballots are very important to the people of Maine,” Ms. Collins said. “I do not support restrictions that make it more difficult for seniors to cast votes on Election Day via the mail.”

Others worry that the uncertainty caused by the court decision, as well as the blitz of other federal actions by Mr. Trump, could diminish voter confidence in the electoral system.

Barbara J. Pariente, a Democratic-appointed former chief justice on the Florida Supreme Court, said that even if it was not yet clear whether Mr. Trump’s order would go into effect, it could potentially cause “citizens to fear that there is something wrong with mail-in voting.”

“Clearly this is voter intimidation,” said Ms. Pariente, a member of the nonpartisan civic organization Keep Our Republic. “I see what’s going on with the Trump administration as one more step in an assault on our democracy.”