Representative Stephen F. Lynch, a longtime South Boston Democrat, got into a contentious back-and-forth with Postmaster General Louis DeJoy on Monday during a hearing on operational changes that have led to mail delays across the country.

Lynch grilled DeJoy on the changes, saying for the first time in the centuries-long running of the post office, “service has been delayed and the mail is piling up,” adding: “You have ended a once-proud tradition.”

In a phone interview following the hearing, Lynch said DeJoy should resign if he doesn’t return 671 mail-sorting machines that were removed from service as part of sweeping operational changes at the Postal Service. And, Lynch said, Democrats are looking into what legal options may be available to “compel” DeJoy to “use those machines in order to process the mail.”

During the earlier hearing, Lynch noted that the Postal Service has performed admirably during several national crises that pre-dated the COVID-19 pandemic.

“Throughout the Postal Service history, there’s been a tradition of reliable delivery,” Lynch said, adding that the service still operated even during the Civil War. “Even at a moment when the country was at war with itself, the mail was delivered. During the First World War and the Spanish flu influenza of 1918 through the Great Depression, millions of people out of work, 1,000 bank failures, the mail was delivered — on time. Even during the Second World War with the threat of Nazi U-boats, international mail was delivered on time.”

The House’s oversight and reform committee hearing comes as the new postmaster general, a donor to President Trump, is slashing costs across the agency. Trump himself has waged an extended assault on voting by mail, leveling repeated and unsubstantiated claims that it is rife for voter fraud.

Lynch also criticized changes imposed by DeJoy this summer have threatened the reliability of mail service both in Massachusetts and across the country.

“In my heart, I’m tempted to ask, after 240 years of patriotic service delivering the mail: How can one person screw this up in just a few weeks?” Lynch said during the hearing. “I understand you bring private sector expertise. I guess we couldn’t find a government worker who could screw it up this fast.”

He continued: ”We can only reach two conclusions: One, either through gross incompetence, you have ended the 240-year history of delivering the mail reliably on time, or the second conclusion. . . is that you’re doing this on purpose, and that you’re deliberately dismantling this once-proud tradition.”

As Lynch was advised that his time was waning, the congressman cut in to ask, “My last question is this: What the heck are you doing?”

As DeJoy began outlining how he was proud of postal workers and noting he was proud to lead the organization, Lynch interrupted to ask, “Will you put the machines back?” Lynch was referring to mail-sorting machines removed from service as part of sweeping operational changes at the Postal Service.

DeJoy did not immediately answer, saying that Lynch’s other comments were “outrageous,” but after Lynch asked him again, the USPS head replied firmly, “No, I will not.”

“Well, there you go,” Lynch said.

“There I go, what?” DeJoy shot back, as the two began talking over each other, leading chairwoman Carolyn Maloney to bang a gavel and call, “Order, order, order, order.”

“The gentleman may answer the question without being interrupted,” she said, referring to DeJoy. “The question is: Will you put the machines back?”

“The answer is no,” DeJoy said. He also denied any wrongdoing and accused Lynch and other Democrats of spreading “inaccurate” misinformation.

The postmaster general also disputed published reports that he has eliminated overtime for postal workers and said a Postal Service document outlining overtime restrictions was written by a mid-level manager. DeJoy, who has said his “No. 1 priority” is to ensure election mail arrives on time, said he will authorize expanded use of overtime, extra truck trips and other measures in the weeks before the election to ensure on-time delivery of ballots.

President Trump also weighed in on the DeJoy hearing Monday.

“All the Radical Left Democrats are trying to do with the Post Office hearings is blame the Republicans for the FRAUD that will occur because of the 51 Million Ballots that are being sent to people who have not even requested them,” Trump tweeted. “They are setting the table for a BIG MESS!”

During the later phone interview, Lynch said the removal of the mail sorting machines was initially contemplated before the pandemic.

“The old plan did not anticipate that voters were going to be asked to stay home and avoid outside contact,” Lynch said, adding that the CDC now advises that “the safest method is to vote by mail” amid the health crisis.

Asked if he felt the changes were an attempt at voter suppression, Lynch said “absolutely, I think so. The president has continually repeated the fact that he believes voting by mail is fraudulent, yet he’s the one who’s voting by mail, him and his family. … He has offered no evidence that there is any fraud going on with the mail.”

The machines that were removed, Lynch said, sort about 32,000 pieces of mail per hour. He said that during visits to Postal Service locations in Brockton and Boston, employees indicated “there’s no way they can process the mail in a timely fashion without the use of those machines.”

At the same time, however, Lynch said he’s confident the American public will have a fair presidential election in November.

“I’ve talked to a lot of the people who work at the post office, mail handlers and letter carriers, and they tell me by hell or high water they are going to deliver this mail,” he said.

Lynch represents Boston, Brockton, Quincy, and several other towns in southeastern Massachusetts.

Later in the hearing, Representative Ayanna Pressley of Massachusetts also sharply questioned DeJoy on how the US Postal Service is keeping its workforce safe in the era of COVID-19, and called on him to resign amid cuts to the agency that have slowed down mail service.

“Mr. DeJoy, the hardworking people of the United States Postal Service deserve a better leader. In my opinion, the only thing you should be delivering is your resignation,” she said.

Christina Prignano and Matt Stout of the Globe staff contributed. Material from the Associated Press was used in this report.