In a letter to the inspector general, Tammy L. Whitcomb, the lawmakers referred to reports that Mr. DeJoy had limited overtime pay and made other changes that critics say have slowed mail delivery and made it less reliable.
The lawmakers urged Ms. Whitcomb to investigate the effect of the changes on the delivery of critical mail, including medication, paychecks and bills.
“These delays also pose a threat to the November election, particularly during the coronavirus pandemic when millions of Americans are expected to choose to vote by mail rather than wait in long lines on Election Day,” the lawmakers wrote.
The Democratic lawmakers who signed the letter were Senators Elizabeth Warren of Massachusetts, Gary Peters of Michigan and Thomas R. Carper of Delaware and Representatives Carolyn B. Maloney of New York, Stephen F. Lynch of Massachusetts, Gerald E. Connolly of Virginia and Brenda Lawrence of Michigan.
Democrats, including those who signed the letter, have charged that Mr. DeJoy is doing the bidding of the president, who has long attacked the Postal Service on Twitter. In his remarks on Friday, Mr. DeJoy rejected that accusation.
“While I certainly have a good relationship with the president of the United States, the notion that I would ever make decisions concerning the Postal Service at the direction of the president, or anyone else in the administration, is wholly off-base,” he said. “I serve at the pleasure of the governors of the Postal Service, a group that is bipartisan by statute and that will evaluate my performance in a nonpartisan fashion.”