The US House of Representatives has passed a bill that would inject $25bn (£19bn) into the Postal Service (USPS) ahead of November's election.
The legislation would also block cuts and changes that critics have said will hamper mail-in voting.
Democratic Speaker Nancy Pelosi recalled lawmakers from the summer recess to vote on the bill, which she said would protect the USPS.
After the vote, President Trump tweeted the measure was a Democrat ballot scam.
"Representatives of the Post Office have repeatedly stated that they DO NOT NEED MONEY, and will not make changes, " said Donald Trump. He has threatened to veto the bill, which is in any case unlikely to make progress in the Republican-controlled Senate.
Senate majority leader Mitch McConnell said the chamber would "absolutely not pass" the bill.
Postmaster General Louis DeJoy said earlier that further cost-cutting measures at the postal service would be suspended until after November's vote.
A slowdown in mail deliveries amid cost-saving measures at USPS has fuelled fears about how one of the oldest and most trusted institutions in the US can handle an unprecedented influx of mail-in ballots due to the coronavirus pandemic.
President Trump strongly opposes mail-in ballots and has repeatedly suggested it could lead to widespread voter fraud despite there being no evidence for this.
The "Delivering for America Act" passed by the House in a rare Saturday sitting includes $25bn of emergency coronavirus funding requested by the USPS's board of governors.
More than a dozen Republicans crossed the floor to vote with their Democratic opponents.
The bill would require the USPS to treat all official election correspondence as first-class mail.
The service would be prohibited until January 2021 from implementing or approving any changes to operations or service levels that would "impede prompt, reliable, and efficient service", including closing or reducing the hours of post offices, removing mail sorting machines and mailboxes, or stopping overtime payments.
"This is not a partisan issue," Democratic Representative Carolyn Maloney, the bill's author, said before the debate. "It makes absolutely no sense to impose these kinds of dangerous cuts in the middle of a pandemic and just months before the elections in November."
.@USPS provides a critical lifeline by providing universal access to medications, supplies and mail for every single American, no matter where you live. In just a few hours, the House will pass my legislation blocking the Trump Admins politicization of this essential service. pic.twitter.com/ElgHpRTbqL
— Carolyn B. Maloney (@RepMaloney) August 22, 2020
End of Twitter post by @RepMaloneRead More – Source