Postmaster General Louis DeJoy, a Republican megadonor to President Donald Trump, is set to testify before the Democrat-controlled House Oversight Committee Monday morning as he faces growing calls to step aside over mail service changes that have dramatically slowed deliveries of lifesaving medications and other key packages across the nation just ahead of the November election.

DeJoy's testimony will come just days after the postmaster general appeared at a Republican-controlled Senate Homeland Security and Governmental Affairs Committee hearing, during which he admitted that his policies at the U.S. Postal Service have slowed the mail but downplayed the long-term impact of the changes.

Under pressure from Democratic lawmakers, DeJoy also said he has "no intention" of returning or replacing mail sorting machines that were removed from post offices across the nation before the postmaster general vowed last week to suspend his policy changes until after the election.

Watch the hearing, which is scheduled to begin at 10:00 am ET:

Internal USPS documents released by the House Oversight Committee ahead of Monday's hearing show that DeJoy's operational changes have done far more harm to mail service standards than the postmaster general has publicly acknowledged.

"To those who still claim there are 'no delays' and that these reports are just 'conspiracy theories,' I hope this new data causes them to re-think their position and support our urgent legislation today," Rep. Carolyn Maloney (D-N.Y.), chairwoman of the House Oversight Committee, said in a statement, referring to a bill the House passed over the weekend to reverse DeJoy's policies and bar any additional changes until the end of the Covid-19 pandemic.

"We have all seen the headlines from every corner of our country, we have read the stories and seen pictures, we have heard directly from our constituents," Maloney added, "and these new documents show that the delays are far worse than we were told."