The only Democratic senator to vote against a motion to dismiss the impeachment case against former President Clinton is urging Republicans to vote in favor of hearing witnesses as part of President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE’s Senate impeachment trial.

Former Sen. Russ Feingold (Wis.) sent a letter to every Senate Republican on Thursday pushing them to uphold their constitutional duty.

“I understand the position you are in and the difficult decisions you must make. I once took the same oath to do impartial justice that you recently swore to uphold,” Feingold wrote.

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“As there was then, there is now only one way to ensure that the American people have confidence in the outcome of the momentous decision ahead of you — and that is to ensure that the process by which you arrive at your decision on the President’s removal is a fair one,” he wrote.



Feingold asked senators to vote in favor of hearing witnesses during the trial, noting that the House was unable to hear from firsthand witnesses during the hearings in the lower chamber.

“As fair-minded jurors, judges, and representatives of the American people, you must hear all the testimony and examine all the evidence possible before casting your vote to either convict or acquit,” he wrote.

Feingold said the need to hear from witnesses is “all the more apparent” in wake of “a deluge of new evidence relating to the allegations set forth” in the the articles passed by the House.

Democrats have been pushing to hear from new witnesses as part of the impeachment trial. Their push was bolstered after a New York Times report about details in an unpublished memoir by former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE was released.

The former White House official reportedly claimed that Trump tied Ukraine aid to investigations into his political opponent, former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE and his son Hunter Biden.

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Additionally, following the articles of impeachment that passed in the House last year, Lev Parnas, a former associate of Trump’s personal attorney Rudy Giuliani Rudy GiulianiDemocrats fear Russia interference could spoil bid to retake Senate Grand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Juan Williams: Breaking down the debates MORE, supplied the committees in the lower chamber with information about communications with Ukraine that Democrats argue corroborates the allegations against Trump.

Parnas was indicted on campaign finance charges late last year.

A vote on whether to allow witnesses is expected Friday.

Republicans voiced confidence Tuesday that they will be able to block witnesses.

If all Democrats vote for the measure, they will need at least four Republicans to join them.