Senate Minority Leader Chuck Schumer on Friday accused the Republican Party of orchestrating the "greatest cover-up since Watergate" as the Senate prepared to debate and vote on whether to allow witnesses to testify in President Donald Trump's impeachment trial.

"This is much bigger than President Trump. Preventing witnesses, evidence, and transparency in President Trump's impeachment trial potentially undermines the Constitution for generations."

—Karen Hobert Flynn, Common Cause

The Senate is widely expected as early as Friday evening to oppose permitting witnesses, given swing vote Sen. Lamar Alexander's (R-Tenn.) announcement late Thursday that he will vote no. Alexander's decision sparked widespread anger and the trending Twitter hashtag #LamarAlexanderIsACoward.

Schumer said during a press conference Friday that if the Senate votes against allowing witnesses, "the president's acquittal will be meaningless."

"This is about truth, and today the Senate will vote on whether witnesses and documents are allowed in this trial," said Schumer. "The importance of this vote is self-evident."

SCHUMER: "If my Republican colleagues refuse to even consider witnesses & documents in the trial, this country is headed towards the greatest coverup since Watergate ... [Trump] will conclude he can do it again, & Congress can do nothing about it. He can try to cheat again." pic.twitter.com/5vNB1tIQ9p — Aaron Rupar (@atrupar) January 31, 2020

The Senate at 1pm ET is scheduled to begin four hours of debate on whether to approve witnesses, followed by a vote. If the Republican-controlled chamber decides against allowing witnesses, it will be the first time in U.S. history the Senate has held an impeachment trial without witness testimony, according to PolitiFact.

An analysis put out this week by Citizens for Responsibility and Ethics in Washington (CREW) detailed how in each of the 15 impeachment cases completed by the Senate over its 231 year history, "witnesses who were not heard during the House of Representatives' impeachment investigations testified in front of the Senate."

"Throughout this impeachment trial, we have heard the unsupported claim that Senators cannot, and should not, consider testimony from witnesses the House had not already heard from. History certainly proves otherwise," said CREW executive director Noah Bookbinder earlier this week. "In the light of the huge new revelations that have come to the public's attention since the start of the trial and the President's efforts to keep witnesses and documents out of the House process, the Senate must now do its constitutional duty and call forward any and all appropriate witnesses to ensure a fair, thorough and impartial trial."

A final vote on whether to acquit or remove Trump could come as early as Friday evening, but anonymous Republican senators and aides told Politico the trial could extend until next week if House Democratic impeachment managers push for more time to make closing arguments.

Just ahead of the Senate debate on witnesses, the New York Times reported that former national security adviser John Bolton—one of the potential witnesses in the trial—alleges in an unpublished book manuscript that Trump instructed him in May of 2019 to help with the "pressure campaign to extract damaging information on Democrats from Ukrainian officials."

Sen. Chris Murphy (D-Conn.) tweeted Thursday night that the Senate will be "disgraced" if it votes to acquit Trump without hearing witness testimony or considering documentary evidence that the White House has withheld from Congress.

"If the trial is rigged to keep hidden the most damning, most important, most relevant evidence, then it's not a trial," wrote Murphy. "Nor is it an acquittal. It's a cover-up."

Progressive advocacy groups on Friday urged the U.S. public to continue calling their senators to demand witnesses:

WE HAVE A NUMBER FOR THAT!

Call the Senate: 855-912-2982 https://t.co/xQAPiHkXBb — MoveOn (@MoveOn) January 31, 2020

Sen. Elizabeth Warren (D-Mass.), a 2020 Democratic presidential candidate, wrote in a pair of tweets Friday morning that "history will judge us for what happens next."

"Faith in our American institutions is at an all-time low," said Warren. "The fact that GOP senators are covering up the president's corruption with a sham impeachment trial without witnesses documents doesn't help."

Americans deserve to know that the President is using the power of his/her office to work in the nation’s interest, not his own personal interest. And they deserve a Senate with the political courage to stand up to a President that has abused his power & corrupted our government. — Elizabeth Warren (@SenWarren) January 31, 2020

Karen Hobert Flynn, president of government watchdog group Common Cause, warned in a letter (pdf) sent to every U.S. senator Thursday that ending the trial without witnesses "could undermine our democracy for generations."

"This is much bigger than President Trump," Hobert Flynn wrote. "Preventing witnesses, evidence, and transparency in President Trump's impeachment trial potentially undermines the Constitution for generations."

"Americans deserve nothing less than the full truth," she added. "They deserve to see a fair trial, and they are watching closely to see if Senate Republicans fulfill their constitutional duty to serve as an impartial jury or blindly conduct a rigged trial."