WASHINGTON — Speaker Nancy Pelosi of California announced Sunday that she would call the House back from its annual summer recess for a vote this week on legislation to block changes at the Postal Service that voting advocates warn could disenfranchise Americans casting ballots by mail during the pandemic.

The announcement came after the White House chief of staff Sunday signaled openness to providing emergency funding to help the agency handle a surge in mail-in ballots, and as Democratic state attorneys general said that they were exploring legal action against cutbacks and changes at the Postal Service.

The moves underscored rising concern across the country over the integrity of the November election and how the Postal Service will handle as many as 80 million ballots cast by Americans worried about venturing to polling stations because of the coronavirus. President Donald Trump has repeatedly derided mail voting as vulnerable to fraud, without evidence, and the issue had become a prominent sticking point in negotiations over the next round of coronavirus relief.

The House was not scheduled to return for votes until Sept. 14, but is now expected to consider a Postal Service bill as soon as Saturday, according to a senior Democratic aide familiar with the plans. Rep. Steny H. Hoyer of Maryland, the majority leader, is expected to announce the final schedule Monday.

“Lives, livelihoods and the life of our American democracy are under threat from the president,” Pelosi said in a letter to Democratic lawmakers. “That is why I am calling upon the House to return to session later this week.”

The abrupt return to Washington was announced just hours after Democrats called on top Postal Service officials Sunday to testify on Capitol Hill this month about recent policies that they warned pose “a grave threat to the integrity of the election.” It also demonstrates the growing alarm over changes the Postal Service is enforcing under its leader, Louis DeJoy, the postmaster general and a Trump megadonor, less than three months before a general election. Some of the changes, which DeJoy describes as cost-cutting measures, include ending overtime pay and the removal or transfer of some sorting machines.

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The move also increases political pressure on Republican senators facing competitive reelection in rural states like Montana and Alaska that are heavily mail dependent, some of which have already expressed unease with delays in mail delivery. (Sen. Susan Collins, R-Maine, called on Senate to return and act on a coronavirus relief package that included funding for the agency.). It is unclear whether the Senate will take up the legislation, which would require the agency to maintain current service standards until Jan. 1, 2021, or until after the pandemic is over.

Sen. Chuck Schumer of New York, the minority leader, demanded Sunday that Sen. Mitch McConnell, R-Ky., the majority leader, bring senators back to Capitol Hill to take up the House measure that he said in a statement “will undo the extensive damage Mr. DeJoy has done at the Postal Service.”

Mark Meadows, the White House chief of staff, claimed Sunday that no mail-sorting machines would be dismantled before Election Day and insisted that the notion that they would be was a false “political narrative by my Democrat colleagues.”

“The president of the United States is not going to interfere with anybody casting their votes in a legitimate way, whether it’s the post office or anything else,” he said, speaking on CNN’s “State of the Union.”

But Democrats point to the Postal Service’s decision to tell states last month that it may not be able to meet their deadlines for delivering last-minute mail-in ballots.

“The postmaster general and top Postal Service leadership must answer to the Congress and the American people as to why they are pushing these dangerous new policies that threaten to silence the voices of millions, just months before the election,” top Democrats said in a joint statement Sunday. DeJoy had been scheduled to appear before the House Oversight Committee in late September, and lawmakers have already requested information about the changes.

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But Pelosi and Rep. Carolyn B. Maloney of New York, chair of the House Oversight Committee, pressed for DeJoy and Robert M. Duncan, chair of the Postal Service Board of Governors, to testify Aug. 24. Schumer and Sen. Gary Peters of Michigan, the top Democrat on the Homeland Security Committee, said Senate Democrats had begun investigating the slowdown in mail deliveries. Peters urged his Republican counterpart, Sen. Ron Johnson of Wisconsin, to hold a similar hearing.

A coalition of Democratic attorneys general are also considering suing the administration over the implications of recent policy changes at the Postal Service for the November election. Washington state is expected to be the first to file this week, and Pennsylvania and New York are likely to follow, according to two people with knowledge of the discussions.

“We are going to make sure that every American’s vote counts this fall, whether cast by mail or in-person,” said Mark R. Herring, the attorney general of Virginia, one of the states considering legal action. “My colleagues and I are working as we speak to determine what Trump and DeJoy are doing, whether they have already violated or are likely to violate any laws, and what tools we have at our disposal to put a stop to President Trump’s ongoing attack on our Postal Service and our democracy.”

The changes under DeJoy, who has significant financial interests in the Postal Service’s rivals and contractors, in addition to Trump’s frequent attacks on the agency, have prompted concerns about its politicization. Since his appointment in May, DeJoy has put in place cost-cutting measures that he says are intended to overhaul an agency beleaguered by billion-dollar losses.

Trump, for his part, has assailed the service near daily, baselessly claiming that the election could be riddled with fraud if voting by mail is widely used. (He also requested an absentee ballot in Florida, public records show.) He has made clear that he opposes providing additional relief to the agency, though he said he would not veto an economic stimulus package over such funding.

Protesters in Washington called over the weekend for the resignation of the postmaster general, saying changes under his purview had jeopardized people’s ability to vote.

About 100 people gathered outside DeJoy’s apartment complex Saturday, according to videos posted on social media. Banging spoons on pots, blaring horns and chanting “resign” in the wealthy residential neighborhood of Kalorama, many in the group were wearing masks and maintaining social distance.

Posts on social media also showed protesters delivering fake absentee ballots to the entrance of DeJoy’s building, cluttering the glass front doors with folded sheets of paper that read: “Save the post office. Save our democracy.”