Unredacted emails published on Thursday show that the order to maintain a freeze on military aid to Ukraine came directly from President Trump, despite Pentagon officials warning him that it might be illegal.
The president was impeached last month by the House of Representatives over allegations that he abused his power and obstructed Congress when he withheld funding to Ukraine in exchange for an investigation into his Democratic rivals, including Senator Joe Biden.
Congress returns from its Christmas break on Friday with Mr Trump's impeachment proceedings in stalemate. In order for the president to be removed from office, Democrats need to instigate a trial in the Senate. Ms Pelosi has so far refused to send the two articles of impeachment against Mr Trump to the Senate until the chamber agrees on trial parameters that she considers fair.
No progress was made over the Christmas break as to the rules of the trial. Democrats in the Senate want to hear from key witnesses who refused to testify during the House investigation, and obtain documents which were denied to the impeachment probe.
Republicans have angrily rejected these plans, with Senate Majority Leader Mitch McConnell admitting last month that he was working in "total co-ordination" with the White House on how to conduct the trial.
The emails published on Thursday by a national security website show that a White House official told the Pentagon that the order to hold the Ukraine aid came from the president himself.
According to Just Security, an August 30 email sent by Michael Duffey, associate director of national security programs at the Office of Management and Budget, to the Pentagon's comptroller said the aid freeze would continue at Mr Trump's direction, despite mounting legal worry within the Defense Department.
Trump engaged in unprecedented, total obstruction of Congress, hiding these emails, all other documents, and his top aides from the American people.
His excuse was a phony complaint about the House process.
What’s the excuse now? Why won’t Trump & McConnell allow a fair trial? https://t.co/S3ZlEJMMDB
— Nancy Pelosi (@SpeakerPelosi) January 2, 2020
"Clear direction from POTUS to continue to hold," Mr Duffey wrote in the email.
The unredacted message was one of several not turned over to House investigators conducting the impeachment inquiry, Just Security reported.
On Thursday, top Senate Democrat Chuck Schumer said Mr Duffey's email "further implicates" Trump and compromises Mr McConnell's push to have a trial without documents and witnesses as sought by Democrats.
Mr Schumer said it was imperative that Duffey and other key figures testify, including Secretary of State Mike Pompeo and acting White House chief of staff Mick Mulvaney.
Mr Schumer said the emails "are a devastating blow to Senator McConnell’s push to have a trial without the documents and witnesses we’ve requested. The American people deserve a fair trial that gets to the truth, not a rigged process that enables a cover-up."
Ms Pelosi said" "Trump engaged in unprecedented, total obstruction of Congress, hiding these emails, all other documents, and his top aides from the American people. Why won't Trump & McConnell allow a fair trial?"
Mr Trump on Thursday repeated his claim that the impeachment effort was a "partisan witch hunt" that has fuelled national divisions.