WASHINGTON – Welcome to the next election battleground: the post office.

President Donald Trump's yearslong assault on the Postal Service and his increasingly dire warnings about the dangers of voting by mail are colliding as the presidential campaign enters its final months. The result has been to generate new concerns about how he could influence an election conducted during a pandemic in which greater-than-ever numbers of voters will submit their ballots by mail.

In tweet after all-caps tweet, Trump has warned that allowing people to vote by mail will result in a "CORRUPT ELECTION" that will "LEAD TO THE END OF OUR GREAT REPUBLICAN PARTY" and become the "SCANDAL OF OUR TIMES." He has predicted that children will steal ballots out of mailboxes. On Thursday, he dangled the idea of delaying the election instead.

Members of Congress and state officials in both parties rejected the president's suggestion and his claim that mail-in ballots would result in widespread fraud. But they are warning that a huge wave of ballots could overwhelm mail carriers unless the Postal Service, in financial difficulty for years, receives emergency funding that Republicans are blocking during negotiations over another pandemic relief bill.

At the same time, the mail system is being undercut in ways set in motion by Trump. Fueled by animus for Jeff Bezos, the founder of Amazon, and surrounded by advisers who have long called for privatizing the post office, Trump and his appointees have begun taking cost-cutting steps that appear to have led to slower and less reliable delivery.

In recent weeks, at the direction of a Trump campaign megadonor who was recently named the postmaster general, the service has stopped paying mail carriers and clerks the overtime necessary to ensure that deliveries can be completed each day. That and other changes have led to reports of letters and packages being delayed by as many as several days.

Voting rights groups say it is a recipe for disaster.

"We have an underfunded state and local election system and a deliberate slowdown in the Postal Service," said Wendy Fields, executive director of the Democracy Initiative, a coalition of voting and civil rights groups. She said the president is "deliberately orchestrating suppression and using the post office as a tool to do it."

Kim Wyman, the Republican secretary of state in Washington, one of five states where mail-in balloting is universal, said Wednesday on NPR's "1A" program that "election officials are very concerned, if the post office is reducing service, that we will be able to get ballots to people in time."

During his eulogy Thursday for Rep. John Lewis, former President Barack Obama lamented what he said was a continuing effort to attack voting rights "with surgical precision, even undermining the Postal Service in the run-up to an election that is going to be dependent on mailed-in ballots so people don't get sick."

Louis DeJoy, the postmaster general, defended the changes, saying in a statement that the ban on overtime was intended to "improve operational efficiency" and to "ensure that we meet our service standards."

DeJoy declined to be interviewed. David Partenheimer, a spokesman for the Postal Service, said that the nation's post offices had "ample capacity to adjust our nationwide processing and delivery network to meet projected election and political mail volume, including any additional volume that may result as a response to the COVID-19 pandemic."

A plunge in the amount of mail because of a recession — which the United States entered into in February — has cost the Postal Service billions of dollars in revenue, with some analysts predicting that the agency will run out of money by spring. Democrats have proposed an infusion of $25 billion. On Friday, House Speaker Nancy Pelosi accused Republicans, who are opposed to the funding, of wanting to "diminish the capacity of the postal system to work in a timely fashion."

Arthur B. Sackler, who runs the Coalition for a 21st Century Postal Service, a group representing the biggest bulk mailers, said the changes were concerning even though his organization did not take a position on voting by mail.

"Like any other mail, this could complicate what is already going to be a complicated process," Sackler said. "A huge number of jurisdictions are totally inexperienced in vote by mail. They have never had the avalanche of interest that they have this year."

Many states have already loosened restrictions on who can vote by mail: In Kentucky, mail-in ballots accounted for 85% of the vote in June's primary. In Vermont, requests for mail-in ballots are up 1,000% over 2018.

Michigan voters had requested nearly 1.8 million mail-in ballots by the end of July, compared with about 500,000 by the similar time four years ago, after the secretary of state mailed absentee ballot applications to all 7.7 million registered voters.

In the suburban Virginia district of Rep. Gerald E. Connolly, a Democrat who leads the House subcommittee that oversees the Postal Service, 1,300 people voted by mail in a 2019 primary — last month, more than 34,000 did.

"We are worried about new management at the Postal Service that is carrying out Trump's avowed opposition to voting by mail," Connolly said. "I don't think that's speculation. I think we are witnessing that in front of our own eyes."

Erratic service could delay the delivery of blank ballots to people who request them. And in 34 states, completed ballots that are not received by Election Day — this year it is Nov. 3 — are invalidated, raising the prospect that some voters could be disenfranchised if the mail system buckles.

In other states, ballots can be tallied as long as they are postmarked by Election Day, but voting rights groups say ballots are often erroneously delivered without a postmark, which prevents them from being counted.

The ability of the Postal Service "to timely deliver and return absentee ballots and their work to postmark those ballots will literally determine whether or not voters are disenfranchised during the pandemic," said Kristen Clarke, president of the National Lawyers' Committee for Civil Rights Under Law.

But Trump — who himself has repeatedly voted by mail in recent elections — has set in motion changes at the Postal Service that could make the problem worse.

A series of Postal Service documents titled "PMGs expectations," a reference to the postmaster general, describe how Trump's new leadership team is trying to cut costs.

"Overtime will be eliminated," says the document, which was first reported by the Washington Post. "Again, we are paying too much overtime, and it is not cost effective and will soon be taken off the table. More to come on this."

The document continues: "The USPS will no longer use excessive cost to get the basic job done. If the plants run late, they will keep the mail for the next day."

Another document, dated July 10, says, "One aspect of these changes that may be difficult for employees is that — temporarily — we may see mail left behind or on the workroom floor or docks."

With the agency under financial pressure, some offices have also begun to cut back on hours. The result, according to postal workers, members of Congress and major post office customers, is a noticeable slowdown in delivery.

"The policies that the new postmaster general is putting into place — they couldn't lead to anything but degradation of service," said Mark Dimondstein, president of the American Postal Workers Union. "Anything that slows down the mail could have a negative impact on everything we do, including vote by mail."

In mid-July, Rep. Carolyn Maloney, D-N.Y., chairwoman of the House Oversight and Reform Committee, and Connolly wrote a letter to DeJoy raising questions about the ban on overtime and the other changes.

"While these changes in a normal year would be drastic," the lawmakers wrote, "in a presidential election year when many states are relying heavily on absentee mail-in ballots, increases in mail delivery timing would impair the ability of ballots to be received and counted in a timely manner — an unacceptable outcome for a free and fair election."

Trump has been assailing the Postal Service since early in his presidency, tweeting in 2017 that the agency was becoming "dumber and poorer" because it charged big companies too little for delivering their packages.

The president has repeatedly blamed Bezos for the financial plight of the Postal Service, insisting that the post office charges Amazon too little, an assertion that many experts have rejected as false.

In the past three years, Trump has replaced all six members of the Postal Service Board of Governors.

In May, the board, which includes two Democrats, selected DeJoy, a longtime Republican fundraiser who has contributed more than $1.5 million to Trump's 2016 and 2020 campaigns, to be postmaster general. According to financial disclosures, DeJoy and his wife, Aldona Wos, who has been nominated to be the ambassador to Canada, have $115,002 to $300,000 invested in the Postal Service's major competitor, UPS.

Two board members have since departed. David C. Williams, the vice chairman, left in April over concerns that the Postal Service was becoming increasingly politicized by the Trump administration, according to two people familiar with his thinking. Ronald Stroman, who oversaw mail-in voting and relations with election officials, resigned in May.

One of the remaining members, Robert M. Duncan, is a former Republican National Committee chairman who has been a campaign donor to Trump.