WASHINGTON - The United States Postal Service (USPS) will be able to deliver the nation's mail-in ballots fully and on time, said Postmaster-General Louis DeJoy on Friday (Aug 21), acknowledging recent delays in mail delivery while defending the cost-cutting changes made under his watch.

Mr DeJoy was grilled at a Senate hearing, which had been called due to growing concerns that the embattled USPS would not be able to cope with the expected surge in mail-in voting due to the coronavirus pandemic.

The furore over the USPS delays shows just how politicised a commonly accepted method of voting has become, and how much President Donald Trump's attacks on voting by mail have created an atmosphere of mistrust over any organisational changes.

Democrats fear a deliberate partisan attempt by Mr DeJoy to sabotage the delivery of ballots to help President Trump, who has repeatedly attacked the legitimacy of mail-in voting, win the election.

Mr DeJoy, the CEO of a logistics company, has donated more than US$1.2 million (S$1.6 million) to the Trump campaign and is the first postmaster-general in decades to come from outside the USPS.

He was appointed to the position in June by a board of governors with ties to Mr Trump.

On Friday, Mr DeJoy called the suggestions that he was interfering with ballot delivery "an outrageous claim" and said he had only once spoken with Mr Trump in a congratulatory call when he first got the post.

Mr DeJoy promised to continue the USPS practice of treating election mail as first-class mail and stressed that he would suspend further changes until after November's election, to avoid even the appearance of any impact on election mail.

But he also said that rolling back the measures taken so far, including the controversial removal of some mailboxes and mail-sorting machines to make space for package sorting equipment, was "not needed".

During the hearing, Democrat and Republican senators expressed concern about widespread delays in mail delivery, which vulnerable seniors and veterans relied on to receive their medicine and essential cheques, particularly in rural areas of America.

Mr DeJoy apologised for the delays, but also said his reforms were needed to keep the USPS financially afloat.

"Managing the Postal Service in an efficient and effective manner cannot succeed if everything is politicised," he said.

He said he supported voting by mail and thought the American public should be able to do so, in contrast to Mr Trump's repeated disparagement of the process.

The President has repeatedly claimed without proof that voting by mail enables widespread electoral fraud and benefits Democrats, a move that has alarmed critics, who view it as an attempt to undermine the legitimacy of the result should he lose.

Hours after Mr DeJoy's testimony, Mr Trump told supporters at the conservative Council for National Policy in Arlington, Virginia that voting by mail "could be one of the greatest catastrophes in the history of our country".

"You'll have double voting, where they'll send in a ballot and then they'll go vote. What are the chances some state is going to be able to catch this?" he said.

In fact, more and more Americans are voting by mail, although this varies widely by state.

The share of voters who vote by mail has steadily risen from 7.8 per cent in 1996 to 20.9 per cent in 2016, according to a Pew Research Centre analysis.

A Pew poll in April also found that 70 per cent of Americans supported allowing any voter to vote by mail if they wanted to, although Democrats were more supportive than Republicans.

Democrats, who fear that Republicans' resistance to make it easier to vote by mail is an attempt to suppress voters, point to Mr Trump's past comments.

He had said he opposed more funding for the USPS to hamper its efforts to expand mail-in voting, which could put his re-election at risk.

"If we don't make a deal, that means they don't get the money," said Mr Trump on Aug 14 in a Fox interview. "That means they can't have universal mail-in voting; they just can't have it."

Republican senator Mitt Romney of Utah, who has emerged as a critic of Mr Trump, told Mr DeJoy at Friday's hearing that concerns about the USPS had to be viewed in light of the President's comments.

Said Mr Romney: "Any surprise at such concerns has to be tempered by the fact that the President has made repeated claims that mail-in voting will be fraudulent, that he doesn't want to give more money to the post office because without more money, you can't have universal mail-in voting."