Speaker Nancy Pelosi is calling the House of Representatives back into session this week to vote on a bill prohibiting the US Postal Service from implementing any changes to operations or level of service.
The action comes amid growing concerns the Trump White House is trying to undermine the agency during the coronavirus pandemic while states expand mail-in voting options ahead of the November election.
In a letter to Democratic politicians on Sunday evening, Ms Pelosi also called on her colleagues to appear at a post office in their district on Tuesday for a coordinated news event.
“In a time of a pandemic, the Postal Service is Election Central. Americans should not have to choose between their health and their vote,” she wrote.
Earlier on Sunday, Democrats demanded that leaders of the Postal Service testify at an emergency oversight hearing on August 24 about mail delays.
The House Oversight and Reform Committee said it wanted to hear from new Postmaster General Louis DeJoy and from the chair of the Postal Service board of governors, Robert Duncan.
The agency did not immediately respond to requests for comment about whether the two men would appear before the House committee.
But it said on Sunday it would stop removing its distinctive blue mailboxes through mid-November, following complaints from customers and members of Congress that the collection boxes were being taken away.
“Given the recent customer concerns the Postal Service will postpone removing boxes for a period of 90 days while we evaluate our customers’ concerns,” said Postal Service spokeswoman Kimberly Frum.
Mr DeJoy, a major Republican donor and ally of the president who took control of the agency in June, has pledged to modernise the money-losing agency to make it more efficient. He has eliminated most overtime for postal workers, imposed restrictions on transportation, and reduced of the quantity and use of mail-processing equipment.
“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,” congressional Democrats said in a statement announcing the hearing.
Mr Trump said last week he was blocking a 25 billion dollar (£19.08 billion) emergency injection sought by the Postal Service, as well as a Democratic proposal to provide 3.6 billion dollars (£2.75 billion) in additional election money to the states.
The Republican president worries that mail-in voting could cost him reelection. The money for the post office is intended to help with processing an expected surge of mail-in ballots. Both funding requests have been tied up in congressional negotiations over a new coronavirus relief package.
“What you are witnessing is a president of the United States who is doing everything he can to suppress the vote, make it harder for people to engage in mail-in balloting at a time when people will be putting their lives on the line by having to go out to a polling station and vote,” said Senator Bernie Sanders.
Mr Trump acknowledged in a Fox Business interview on Thursday that he is starving the agency of money to make it harder to process an expected surge of mail-in ballots.
Funding a cash-strapped Postal Service has quickly turned into a top campaign issue as Mr Trump presses his unsupported claim that increased mail-in voting will undermine the credibility of the election and Democrats push back.
Mr Trump, who spent the weekend at his New Jersey golf club, derided universal mail-in voting as a “scam” and defended Mr DeJoy as the right person to “streamline the post office and make it great again”.