Speaker Nancy Pelosi will call on the House of Representatives to return to session in the coming days to vote on a bill to protect the US Postal Service.
Ms Pelosi has accused President Trump of a “campaign to sabotage the election”.
“Lives, livelihoods and the life of our American Democracy are under threat from the president,” she added.
Ms Pelosi said she would call on House representatives to vote on a new bill to prohibit the USPS from introducing any changes to the service or operations it provided at the beginning of this year in the coming week. A date for the vote has not yet been announced.
It comes after the USPS warned that millions of mail ballots may not arrive in time to be counted in the election.
Critics blamed the new USPS head – a loyal supporter of the president – for a slowdown in deliveries.
A record number of people are expected to vote by mail ahead of the 3 November presidential election due to the pandemic.
The president has repeatedly said mail-in ballots will lead to voting fraud and give a boost to his rival Democrat Joe Biden. However, experts say the mail-in voting system, which Mr Trump himself uses, is safe from tampering.
President Trump has previously told Fox News he was blocking additional funding for the financially troubled agency, because he opposes mail-in voting.
“Now they need that money in order to make the post office work so it can take all of these millions and millions of ballots,” he said. “Now, if we don’t make a deal, that means they don’t get the money. That means they can’t have universal mail-in voting, they just can’t have it.”