President Biden on Wednesday announced three nominees to fill vacant seats on the Postal Service board of governors a move to increase Democratic influence

The president nominated three people to the beleaguered agency’s board on a day when the postmaster general, Louis DeJoy, a major Republican donor, faced questioning on Capitol Hill.

WASHINGTON — President Biden on Wednesday announced three nominees to fill vacant seats on the Postal Service’s board of governors, a move to increase Democratic influence on the future of the beleaguered agency.

If the nominees are confirmed by the Senate, Democrats and Democratic appointees would gain a majority on the nine-member board. That would give them the power to oust Louis DeJoy, a major Republican donor who has served as postmaster general since last year, should they decide to. The board, not the president, hires and fires the postmaster general.

The announcement came on the same day that the House Committee on Oversight and Reform held a hearing on how to address the post office’s widespread service and financial problems, marking the first time that Mr. DeJoy had testified before lawmakers since the election in November.

The Postal Service catapulted to the national spotlight last summer amid nationwide slowdowns that coincided with operational changes instituted by Mr. DeJoy, raising fears ahead of the election about vote-by-mail delays. Democrats accused Mr. DeJoy, a supporter of President Donald J. Trump, of trying to undercut mail balloting at a time when Mr. Trump was also promoting a false narrative that it was rife with fraud.

But Mr. DeJoy has also drawn fire for continued delivery problems since the election, as the Postal Service struggles to find a sounder financial footing.

In his opening statement on Wednesday, Mr. DeJoy offered an apology for the service’s slow delivery times during the 2020 holiday season.

“We must acknowledge that during this peak season, we fell far short of meeting our service targets,” he said. “Too many Americans were left waiting for weeks for important deliveries of mail and packages. This is unacceptable, and I apologize to those customers who felt the impact of our delays.”

He promised that the agency would “do better” and added, “Above all, my message is that the status quo is acceptable to no one.”

Mr. Biden’s announcement was his most direct action to date to address the service’s problems. The president’s nominees are Anton Hajjar, the former general counsel of the American Postal Workers Union; Amber McReynolds, the chief executive of the National Vote at Home Institute; and Ron Stroman, who resigned last year as deputy postmaster general and later served on Mr. Biden’s transition as the leader of the agency review team for the Postal Service.

“These experienced and tested leaders will ensure the U.S.P.S. is running at the highest of service standards and that it can effectively and efficiently serve all communities in our country,” the White House said in its announcement.

Mr. DeJoy said Postal Service leaders had been developing a 10-year strategy for the agency that would include “a commitment to six- and seven-day-a-week delivery service to every address in the nation.”

He later acknowledged, however, that the Postal Service was “evaluating all service standards,” suggesting that it might not be able to meet its current benchmarks for timely mail delivery.

Lawmakers are debating a bill that would repeal a financially burdensome requirement that the service pre-fund its retiree health care, among other provisions. Both Republican and Democratic lawmakers have expressed some support for the changes.

But during the hearing, Republicans reminded the committee of the political sparring last summer amid the delivery delays, calling out Democrats for what they saw as unfair accusations levied against Mr. DeJoy.

“Why should we believe that the rabid resistance is not going to continue?” said Representative Jody B. Hice, Republican of Georgia. “If moving blue boxes and mail sorters and trying to bring sanity to overtime usage is somehow viewed as criminal activity by the postmaster, then what in the world is going to happen to the business plan that he comes up with?”

The delays last year prompted a slew of lawsuits that forced the Postal Service to temporarily postpone the operational changes. But service issues have continued to plague the agency, and some Democrats have called for Mr. Biden to replace the entire Postal Service board.

Asked by Representative Jim Cooper, Democrat of Tennessee, how long he planned to serve in his post, Mr. DeJoy responded: “A long time. Get used to me.”

At another point, Mr. DeJoy said Mr. Biden had not called on him to resign, nor had any members of the board.

Representative Bill Pascrell Jr., Democrat of New Jersey, praised the president’s announcement on Wednesday, saying in a statement that it was an “important step, and I hope only the beginning.” But Mr. Pascrell added that Mr. Biden should remove the existing board members, whom he said had “been silent and complicit to the DeJoy sabotage.”

On Tuesday, the Postal Service chose Oshkosh Defense, a manufacturer of military vehicles, for a $482 million deal to provide the next generation of postal delivery trucks, over an electric-vehicle maker.