U.S. Sen. Cory Gardner said Wednesday that he thinks the Senate has heard from enough witnesses in President Donald Trump's impeachment trial, taking off the table a potential Republican vote to subpoena national security adviser John Bolton.

“I do not believe we need to hear from an 18th witness," the Colorado Republican told Colorado Politics in a statement. "I have approached every aspect of this grave constitutional duty with the respect and attention required by law, and have reached this decision after carefully weighing the House managers and defense arguments and closely reviewing the evidence from the House, which included well over 100 hours of testimony from 17 witnesses.”

Gardner had previously been noncommittal about Democrats' demands to call more witnesses, including Bolton, who writes in a forthcoming book that Trump told him he withheld military aid from Ukraine to pressure the country to investigate Democratic political rival Joe Biden, according to The New York Times.

It will take 51 votes to call witnesses or subpoena documents, meaning four Republicans will have to join the 47 Senate Democrats and independents who have said senators should consider more evidence before deciding whether to remove Trump on charges he abused his power and obstructed an ensuing congressional investigation.

U.S. Rep. Jason Crow, an Aurora Democrat and one of the seven House managers prosecuting the impeachment charges, said Tuesday that he believes Bolton should be called to testify to "make sure people have the full picture of what has happened."

“The president deserves a fair trial. The country deserves a fair trial. The Senate deserves a fair trial,” Crow said in a call with reporters. “And this can’t be the first impeachment trial in American history where there are no witnesses called and no document subpoenaed. That needs to be done, and people deserve nothing less.”

So far, three Republican senators — Susan Collins of Maine, Lisa Murkowski of Alaska and Mitt Romney of Utah — have said they want to hear testimony from witnesses as the impeachment trial enters its next phase. Senators expect to spend Wednesday and Thursday questioning Trump's legal defense team and House Democrats who have been prosecuting the impeachment case and.

"A trial without witnesses and evidence is not a trial at all," tweeted former Colorado Gov. John Hickenlooper, the front-runner in the Demoratic primary for Gardner's seat, on Wednesday morning, linking to the Colorado Politics' report on Gardner's witness position.

A national Quinnipiac University poll released Tuesday found majority support for allowing witnesses to testify in the trial, with 75% of registered voters and 49% of Republican voters saying the Senate should call witnesses.

Gardner, who faces a tough bid for re-election this fall, has been considered among a small number of GOP swing votes on the question, but until Wednesday's statement to Colorado Politics he had remained mum on his intentions.

Tuesday afternoon, after Trump's legal defense team finished its final day of argument, Gardner declined to say whether he wanted to hear additional testimony.

“I have approached every aspect of this grave constitutional duty with the respect and attention required by law, and with the seriousness our oath requires,” Gardner told Colorado Politics in a statement. “Now that the House and defense have closed their arguments, I will continue to closely review the law and evidence presented to the Senate, including testimony from the impeachment witnesses."

Added Gardner: "I am now focused on the questions I will be pursuing during the time allotted for senators’ questioning.”

The Wall Street Journal reported Tuesday afternoon that Gardner was among a small group of Republican senators in competitive states who raised the political implications of extending the impeachment trial during a closed-door GOP caucus meeting.

Gardner told his colleagues that "a longer trial would lead to more Democratic attacks," The Journal reported, citing a Gardner campaign spokesman who later told Colorado Politics that the senator was describing what Democrats have been saying since the trial began.

The Washington Post reported Tuesday night that Gardner and North Carolina Sen. Thom Tillis "indicated during the meeting that they were ready to vote against witnesses and proceed to the final vote," citing anonymous sources said to be familiar with the discussion.

A Gardner campaign spokesman told Colorado Politics that Gardner wasn't linking his vote on additional testimony to his re-election prospects.

Gardner spokesman Jarrod Dobkin said Gardner isn't happy that Democrats' efforts to politicize the impeachment trial are stoking division but insisted, "He still takes his constitutional oath seriously and will vote based on the facts and not anything else like the potential of making the trial longer."

This developing story has been updated.