U.S. Sen. Cory Gardner remained noncommittal Tuesday on whether former national security adviser John Bolton should be subpoenaed in the impeachment trial of President Donald Trump.

The Yuma Republican, one of 100 senators who will likely decide whether to subpoena Bolton this week, said he is reviewing trial testimony and the law. He did not say if he would like to see Bolton or any other witnesses testify.

“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 said in a statement to The Denver Post. “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.”

Gardner said his focus is now on the questions he will ask of trial participants. Senators will have up to 16 hours Wednesday and Thursday to ask questions of the prosecution and defense.

Increased calls for Bolton to be called follow the leak of details from the former national security adviser’s upcoming book, in which he writes that Trump withheld military aid from Ukraine to compel that Eastern European ally to investigate the Bidens, as impeachment prosecutors have alleged.

“Let’s bring Bolton in, let’s have questions and cross-examination, and let’s make sure people have the full picture of what has happened,” said Rep. Jason Crow, an Aurora Democrat and prosecutor in the impeachment trial, during a conference call with reporters Tuesday morning.

The approval of 51 senators will be needed to subpoena Bolton if the matter comes up for a vote later this week. The 47 Senate Democrats and independents in favor of subpoenaing him have sought the support of four Senate Republicans to reach that magic number of 51.

Throughout January, Gardner has faced pressure to be one of those four Republicans but has never indicated he will be.

Complicating matters is Bolton’s political action committee, which has spent millions of dollars in recent years supporting Republicans. The PAC donated $10,000 to Gardner’s 2020 re-election campaign last fall.

“There’s a broad coalition of people supporting Senator Gardner’s re-election, including President Trump, and the Bolton PAC supports Senator Gardner because of his commitment to a strong foreign policy,” said Gardner’s campaign spokesman, Jerrod Dobkin. “All of Senator Gardner’s supporters know what’s at stake in this election and are standing with him in his fight to defeat the far-left, radical proposals his opponents are campaigning on.”

Majority Forward, a nonprofit affiliated with a Democratic political action committee, is now broadcasting two ads that criticize Gardner on impeachment. Republicans for the Rule of Law also has an ad calling for him to subpoena Bolton, and has a billboard in south Denver saying the same. On Wednesday afternoon, liberal activists will gather on the steps on the Colorado Capitol to urge Gardner to vote for witnesses.

Quinnipiac University released a national poll Tuesday that showed 75% of registered voters, including 49% of Republicans, believe the impeachment trial should have witnesses. The poll of 1,905 voters was conducted Jan. 22-27 and had a margin of error of 2.3%.

Gardner has said that commenting on the mechanics of the trial would violate his oath to be an impartial juror, even as other senators, Democrat and Republican, have freely stated their opinions on witnesses. Sen. Michael Bennet, a Denver Democrat, believes Bolton should testify.

“The real problem here is that House Democrats didn’t do their job,” Rep. Ken Buck, a Windsor Republican, said Tuesday on Fox Business. “They didn’t go to court, they didn’t force witnesses into the House. So, we don’t know exactly what’s going to be said if Senate witnesses are called.”

“If the Senate does call witnesses, they should not do a one-for-one trade on witnesses,” Buck said, referring to a proposal to let Republicans subpoena one witness, such as Hunter Biden, for each witness Democrats subpoena, such as Bolton. Buck believes Republicans were not allowed enough witnesses during the House’s impeachment investigation.

Crow opposes a so-called witness swap, but acknowledges the matter is for senators to decide. He accused White House lawyers of trying to “smear” Hunter Biden and his father, Joe Biden, a top Democratic presidential contender, by focusing on the younger Biden’s position with Burisma, a Ukrainian gas company.

“It’s well established that the Burisma allegations are Russian propaganda,” Crow told reporters Tuesday, “and I think it’s really important this not become a political circus or a way for the president to advance his political agenda.”