Senate Minority Leader Charles Schumer Chuck SchumerVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Graham signals support for confirming a Supreme Court nominee this year Pelosi orders Capitol flags at half-staff to honor Ginsburg MORE (D-N.Y.) on Thursday called on the White House to declassify and make public a letter from an aide to Vice President Pence that was admitted as evidence in the impeachment trial.

“I’ve seen the supplemental testimony” from Pence aide Jennifer Williams, Schumer told reporters during a break in the trial. “I’m not sure it should be classified. I agree with the House managers to declassify it.”

He declined to comment further because the document was classified.

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Schumer became just the latest Democrat to push the White House to declassify the mysterious Williams letter. During opening arguments Wednesday night, Rep. Zoe Lofgren Zoe Ellen LofgrenTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence DHS opens probe into allegations at Georgia ICE facility House passes legislation to boost election security research MORE (D-Calif.), one of the House impeachment managers presenting the case against President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE, urged declassification, declaring that “a cover-up is not a proper reason to classify a document.”

And after viewing the document in a secure Senate briefing room Thursday morning, several Democratic senators demanded the document be declassified.

"There's no reason that it should be kept classified. It should be made public, regardless of which side it helps. The American people should judge," Sen. Richard Blumenthal (D-Conn.), a former Connecticut attorney general, told reporters.

The one-page document focuses on a Sept. 18 phone call between Pence and Ukrainian President Volodymyr Zelensky a week before Zelensky met with Trump at the United Nations.

Williams, a special adviser to Pence on Europe and Russia issues, sent the letter to House impeachment investigators after she testified publicly before the House Intelligence Committee in November. During that appearance, Williams described listening in on the now-infamous July 25 phone call between Trump and Zelensky that prompted a whistleblower complaint against the president and eventually led to his impeachment last month.

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Senators on Thursday were extremely tight-lipped about the contents of the Williams letter, but many Democrats began reviewing it behind closed doors after a rare deal was struck a day earlier between the two parties to admit it as evidence into the impeachment trial. Supreme Court Chief Justice John Roberts announced the terms of the deal late Wednesday night.

Rep. Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.), the Intelligence Committee chairman who led the impeachment investigation and now is lead impeachment manager in the trial, has previously stated that declassifying the letter would “provide the public further understanding of the events in question.”

But Trump’s allies are downplaying the significance of the Williams letter.

“I don’t know that any other documents are going to make a more compelling case for my Democratic colleagues at this point,” said Rep. Mark Meadows Mark Randall MeadowsTrump reacts to Ginsburg's death: 'An amazing woman who led an amazing life' Trump carries on with rally, unaware of Ginsburg's death United Airlines, unions call for six-month extension of government aid MORE (R-N.C.), a top Trump defender and surrogate on impeachment who’s been watching the trial.

“For the vast majority of Americans, it’s time to get this [impeachment] over with.”

John Kruzel contributed.