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.
In a letter released on Sunday, Ms Pelosi accused President Trump of a “campaign to sabotage the election”.
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.