WATCH LIVE: Trump's postmaster Louis DeJoy faces Senate grilling over mail delays
Postmaster General Louis DeJoy (R) arrives for a meeting with House Speaker Nancy Pelosi, D-CA, Senate Minority Leader Chuck Schumer, D-NY, Treasury Secretary Steven Mnuchin and White House Chief of Staff Mark Meadows at the US Capitol in Washington, DC on August 5, 2020. (Photo: Mandel Ngan/AFP)

US Postmaster General Louis DeJoy will testify before the Senate Friday on reforms that have fueled an uproar over mail-in voting amid the COVID-19 pandemic, after Donald Trump said he wanted to weaken the agency.

The US president -- who is trailing Democratic challenger Joe Biden in the polls -- has been accused of trying to undermine democracy after he said he was opposed to more funding for the cash-strapped USPS, acknowledging the funds would be used to help process votes.

He has also repeatedly and baselessly linked mail-in voting to fraud at a time when a large share of voters are expected to use absentee ballots as a safer option amid the pandemic.

DeJoy, a Trump donor who took over the position in June, in recent weeks ordered the removal of collection boxes and processing equipment as well as cutting overtime, which a union leader told AFP has slowed down delivery times nationwide.

He also reshuffled senior management, and the USPS warned most states that it could not guarantee on-time delivery of mail in ballots.

Democrats in Congress have seized on the changes and remarks by the president to claim the White House was plotting to undermine confidence in the post office and help Trump's bid for a second term.

"Pure Trump. He doesn't want an election," Biden has said.

DeJoy, targeted by protesters and coming under intense pressure by Congress, reversed position last week, saying the reforms would be put on hold until after the November election.

But Democrats have said the moves do not go far enough, and Democratic Speaker Nancy Pelosi called the House back into session to address the issues.

DeJoy is set to testify before committees in both chambers Friday and next week.

Democratic vice presidential nominee Kamala Harris, a senator and former prosecutor known for her tough questioning, was not expected to appear at Friday's Senate hearing, but will send written questions, CNN reported.

A North Carolina logistics executive, DeJoy has donated $1.2 million to Trump's campaign and almost $1.3 million to the Republican party since 2016, according to The New York Times.

The US is home to the world's worst coronavirus outbreak and Americans are predicted to rely on vote-by-mail in record numbers, with an estimated three-quarters of the population able to do so this fall.