Gardner confirmed his stance in a statement to Colorado Politics on Wednesday.
The electorally vulnerable senator, who is up for reelection in 2020, had been viewed as a potential “yes” vote on the issue. Four Republicans would have to side with Senate Democrats to reach the 51 votes needed to call witnesses.
With Gardner out, that leaves Sens. Susan Collins (Maine), Mitt Romney (Utah) and Lisa Murkowski (Alaska) as the most likely remaining Republican “yes” votes.
Democrats have pushed for the Senate to call witnesses, focusing in particular on former national security adviser John Bolton. A leaked draft of Bolton’s forthcoming book lays out his recollection of President Donald Trump admitting that he was tying the release of aid withheld from Ukraine to that country investigating his political rival Joe Biden.
Bolton earlier chose not to comply with a request to testify during the impeachment investigation in the House, where Democrats hold a majority.
Should the Republican-controlled Senate decline to call any witnesses, it would represent a sharp break from historical precedent. President Andrew Johnson’s Senate trial involved 25 witnesses for the prosecution and 16 for the defense, while President Bill Clinton’s trial had three.
The Colorado Democratic Party blasted Gardner’s decision.
“Today, after getting caught pushing his colleagues to not call for witnesses, Senator Gardner confirmed what we’ve known all along: he never wanted a fair trial, he was never an impartial juror, and he never wanted to get to the truth,” spokesman David Pourshoushtari told HuffPost in an emailed statement.
“The Senate holding a trial without direct witnesses or key documents isn’t a trial ― it’s a cover up, and Senator Gardner is complicit,” he added. “Instead of upholding his oath, Senator Gardner viewed impeachment as a political calculation, and the people of Colorado will not forget.”