NEW YORK (BLOOMBERG) - Barack Obama says President Donald Trump is trying to "actively kneecap the Postal Service" in order to hurt vote-by-mail in November.

In an interview with his former adviser, David Plouffe, the former president pointed to Trump's recent remarks linking his opposition to US$25 billion (S$34 billion) in emergency USPS funding to his fears about mail-in ballots.

"What we've never seen before is a president say, 'I'm going to try to actively kneecap the Postal Service to encourage voting, and I will be explicit about the reason I'm doing it.' That's sort of unheard of, right?" Obama told Plouffe on Cadence13's Campaign HQ podcast.

The vast majority of states are set to allow mail-in voting in response to the coronavirus pandemic.

On Friday, New Jersey Governor Phil Murphy said he would order the November general election to be conducted mostly through the mail.

Obama, who has only spoken out rarely about Trump's actions in office, urged voters and elected officials to take steps to "protect the integrity" of the election.

Trump has repeatedly made unfounded claims that mail-in voting is rife with fraud and hurts Republican candidates, but his opposition has not stopped most states from expanding their vote-by-mail options for the November election.

On Thursday, Trump said that not approving a financial lifeline sought by Democrats would mean that "you can't have universal mail-in voting because they're not equipped to have it."

The Postal Service recently sent letters to 46 states and the District of Columbia warning that mail-in ballots might not arrive in time to be counted, the Washington Post reported on Friday (Aug 14).

In a letter to Pennsylvania's secretary of state disclosed on Thursday, the general counsel for the Postal Service said the state's deadline for requesting to vote-by-mail does not give it enough time to deliver and return a ballot in time to be counted.

The letter says both voters and elected officials should give at least seven days' leeway for a ballot to go through the mail, but state law allows voters to apply to vote by mail by 5pm the Tuesday before Election Day, leaving just six days.

By that standard, more than 20 other states that allow mail-in ballot requests in the days just before the election would not leave enough time for even a one-way mailing, according to state laws compiled by the National Conference of State Legislatures.

Pennsylvania officials cited the letter in a request for a judge to extend the deadline for the state to accept mail-in ballots, which is currently Election Day.

"The Postal Service's announcement represents a significant change to the outlook for voting by mail in the general election," the state said in a legal filing.

But prior to the letter, "the Postal Service had not indicated the likelihood of widespread, continuing, multiple-day mail-delivery delays presenting an overwhelming, statewide risk of disenfranchisement for significant numbers of voters utilising mail-in ballots."

The Postal Service advised the state that voters should request absentee ballots at least 15 days before Election Day and mail the completed ballots at least one week before the state's due date, according to a copy of the letter filed with the court.