Supporters of the Labor Department’s fiduciary rule are calling an 18-month implementation delay too long, while opponents are saying the postponement is needed to give the agency time to review the rule.

In a brief filed in a Minnesota lawsuit Wednesday, the DOL indicated it had submitted to the Office of Management and Budget a proposal to delay the remaining parts of the rule from Jan. 1, 2018, until July 1, 2019. The OMB must review and approve the proposal before it can go into effect. The delay itself could require its own rulemaking process.

The DOL delay proposal could be posted on the OMB’s regulatory review list by Thursday morning.

The proposed postponement is commensurate with the delays that were sought by financial industry opponents of the rule, some of whom asked for an 18-month time-out, but supporters balked.

“The 18 months is excessive,” said Duane Thompson, senior policy analyst at Fi360, a fiduciary duty consulting and credentialing firm. “It appears the Department of Labor has bent over backwards to help industry opponents push off the rule to the last possible moment.”

Barbara Roper, director of investor protection at the Consumer Federation of America, said that opponents of the rule have failed to support the need for a delay through comments that were recently submitted to the agency.

“They have not made a case in real, verifiable data that a delay is justified — let alone a delay of 18 months,” Ms. Roper said. “The only thing that’s preventing retirement savers from receiving the full benefit of the rule is continued uncertainty over its fate since the election. Proposing a delay of this length only reinforces that message.”

The Securities Industry and Financial Markets Association supports the delay.

“We think that this is good news,” said Lisa Bleier, SIFMA managing director and associate general counsel. “[A delay] is necessary to avoid mass confusion if the other aspects of the rule go into effect and are changed later. Hopefully, it’s a clean 18-month delay. That would provide stability for the industry and our clients.”

A DOL spokeswoman was not immediately available for comment.

The measure, which would require all financial advisers to act in the best interests of their clients in retirement accounts, is undergoing a reassessment ordered by President Donald J. Trump that could lead to changes.

“A delay is necessary to provide sufficient time for the department to complete the examination described in the president’s February 3, 2017, memorandum, determine next steps, and announce the results of the examination and its future plans,” James Szostek, vice president, taxes and retirement security at the American Council of Life Insurers, said in a statement.

Two provisions of the rule were implemented in June. The remaining parts include the so-called best-interest-contract exemption that allows brokers to charge variable compensation for products as long as they sign a legally binding agreement to put their clients’ interests ahead of their own. The 18-month delay would apply to the BICE as well as exemptions for principal transactions and for insurance agents and brokers.

The delay gives the DOL time to conduct its review and would allow the Securities and Exchange Commission to get involved in the issue, according to Josh Lichtenstein, an associate at the law firm Ropes & Gray.

“It’s a game-changer,” Mr. Lichtenstein said. “[DOL] could spend a year working on new exemptions and still have time to go through the 60-day notice and comment period. It also opens up the possibility of the SEC moving forward on its own project in coordination with the Department of Labor.”

That’s what Dale Brown, president and chief executive of the Financial Services Institute wants to see.

“This proposed delay represents an important step in protecting Main Street Americans’ access to retirement planning advice, products and services,” Mr. Brown said in a statement. “While the delay is significant, it is critical that the DOL uses the 18 months to coordinate with regulators, in particular the SEC, to simplify and streamline the rule.”

Supporters of the rule say that it mitigates broker conflicts of interest that lead to the sales of inappropriate, high-fee investments that erode savings. They fear that the Trump administration will neuter the regulation.

“It’s a very clear attempt at death by delay,” said Andrew Stoltmann, a Chicago securities attorney and an official with the Public Investors Arbitration Bar Association. “The DOL, with Trump’s likely blessing, is trying to delay and gut the fiduciary duty rule.”