The District of Columbia Public Service Commission on Wednesday breathed new life into Exelon Corp.’s $6.4 billion bid to merge with Pepco, changing course after flatly rejecting the deal last summer.

The three-member commission voted unanimously to consider a settlement that D.C. Mayor Muriel E. Bowser (D) reached this month with Exelon that included $78 million in new benefits, an increase in ratepayer assistance, solar energy subsidies and job guarantees.

The PSC’s decision, announced by Chairman Betty Ann Kane, allows Exelon to update its application instead of starting the time-consuming process over again, a delay that might have jeopardized the deal.

The PSC’s expedited timetable calls for concluding public input by Dec. 18. Exelon had feared a schedule that could have taken up to 18 months.

In a joint statement, Exelon and Pepco said they were pleased with the decision and the schedule, allowing for a decision by the end of March.

“The schedule affords all parties and the public a fair opportunity to present their positions and ensures that the commission has a complete record to render its decision,” the companies said.

Although the PSC’s decision does not guarantee that regulators will approve the deal, opponents quickly voiced their displeasure.

“District residents deserve better. Nevertheless, we are confident this rushed process will provide us enough time to explain how the smoke and mirrors of Mayor Bowser’s secret deal are still not in the public interest,” said Anya Schoolman, president of DC Solar United Neighborhoods, a group that opposes the merger.

Given the size of the merger, the proposal is being watched by environmentalists, utilities and ­public service attorneys, and financial analysts across the country.

Some analysts said that the PSC’s decision, if completed, aids Exelon’s strategy to diversify its business to include more reliable earnings from regulated utilities such as Pepco.

Exelon owns more nuclear plants that any other U.S. utility, and it is trying to broaden its business.

In its rejection last summer, the panel issued a broad ruling that simply said the deal was not in ratepayers’ best interests. The regulators feared that Pepco would become a second-tier company and that a merged entity could inhibit the District’s progress toward alternative energy sources.

In a 30-page order issued late Wednesday, the PSC did not comment on the merits of the mayor’s settlement with the utilities, but justified allowing rehearing, saying it felt “the interest of justice lies in the expeditious resolution of this Merger proceeding through the conduct of a fair and transparent proceeding that will determine whether the Settlement Agreement that has been advanced by several major parties to this proceeding is in the public interest.”

As part of its $78 million commitment to ratepayers engineered by Bowser, Chicago-based Exelon has agreed to spend $17 million on alternative energy, including $3.5 million to support D.C. solar projects and a commitment to buy 100 megawatts of wind power.

The utilities also promised improvements in reliability and technology, and they would relocate 100 jobs into the District. In addition, Exelon-Pepco would hire 102 union workers and protect the current Pepco workforce for five years.

The utilities and their supporters, spearheaded by the Federal City Council and its chief executive, former D.C. mayor Anthony A. Williams, mounted a campaign that delivered strong support for the deal from the business, philanthropic and political communities.

“We applauded the hard-fought settlement negotiated by Mayor Bowser as a good deal,” Williams’s office said in an e-mail. “There is a wave of support with the majority of the D.C. Council and many original opponents of the merger now behind the settlement. This decision by the PSC is another step in the right direction.”