On Monday, the full U.S. Senate convenes for the first time in 2020 with a historic question before it: how best to conduct a trial of the president of the United States.

As it awaits the transfer of articles of impeachment from the House, the Senate must determine whether the trial of President Donald Trump for abuse of power and obstruction of Congress should be quick or lengthy, and who will testify. Colorado’s Democratic senator has been outspoken on both questions. Its Republican senator, Cory Gardner, has not.

Gardner’s every move is being closely watched as calendars flip to 2020, a year that will decide his political future. And in the Senate, where impeachment rules will require a simple majority vote, he can play the role of decider within the narrow Senate Republican majority. But he and his office have not answered questions about his impeachment preferences.

Gardner’s silence dates back months. His public appearances, never numerous in 2019, were rarer still this fall. He has avoided conservative talk radio, once a political safe space, along with most news media. His office agreed to arrange an interview with The Denver Post in Washington, D.C., during the House impeachment process, but later said he was unavailable and instead emailed a statement criticizing that process.

Asked last week whether Gardner believes Trump cabinet members should testify during the trial and whether he agrees with Majority Leader Mitch McConnell’s decision to work in “total coordination” with the White House, Gardner’s spokeswoman sent the same statement, answering neither question.

“Senator Gardner believes Nancy Pelosi’s impeachment inquiry to appease the far-left has been a total circus that has only served to divide this country. Senator Gardner will be a juror and unlike what has happened in the House, he is confident the process in the Senate will be bipartisan and fair,” that statement says.

The last time the senator spoke with a significant contingent of Colorado reporters was Oct. 10, when he made national headlines for refusing to say whether it’s inappropriate for an American president to ask a foreign government to investigate a political rival. Gardner declined to answer the question a dozen times that day.

“What we know is this: (Speaker) Nancy Pelosi and the House of Representatives tried to start impeaching the president years ago. They said it. They talked about it. This is part of a campaign plan that they want to execute. This is a very serious issue. I take it that way, and that’s why we have to have an investigation,” the senator said at the time.

Two weeks later, he co-sponsored a Senate resolution that called the inquiry into Trump’s actions in Ukraine “a political circus.” That resolution called for “a fair and transparent” impeachment process.

The House Intelligence Committee conducted an investigation in November, and the House Judiciary Committee approved articles of impeachment in December. Later that month, the president was impeached for abuse of power and obstruction of Congress.

Throughout that process, as impeachment gripped the nation’s capital, Gardner was quiet. And since then, as senators have debated how to conduct a trial of Trump, he has remained quiet. His critics on the political left and political right have not.

“Instead of ensuring the facts get a fair hearing, Senator Gardner is enabling Mitch McConnell’s partisan coverup while ignoring the rule of law,” said Morgan Carroll, chair of the Colorado Democratic Party.

“Senator Gardner has already compromised his impartiality by taking campaign cash from the defendant, but allowing President Trump to hide critical documents and block testimony from key witnesses would show yet again that Gardner puts his loyalty to the president ahead of getting the truth and doing his job for the people of Colorado.”

Republicans for the Rule of Law bought a billboard on Interstate 25 in Douglas County to accuse Trump of hiding his wrongdoing by preventing witnesses from speaking out. “Sen. Gardner, the witnesses must testify,” that billboard stated. The group also aired a television ad on pro-Trump Fox News shows over the holidays with a similar message for Gardner.

“Gardner has chosen, probably wisely, to try to be as publicly quiet as possible about various issues, including Trump, impeachment and other important political issues of the day that are at all related to Trump,” said Kyle Saunders, a professor of political science at Colorado State University.

“However, keeping that distance is going to get harder and harder,” he added. “Avoiding the spotlight for as long as possible in this tough situation makes sense, but that spotlight is coming, and he will have to somehow thread 20 needles with a length of rope.”

Gardner faces tough re-election odds in November. Nine Democrats are currently competing for the chance to face him, and all nine supported impeachment. John Hickenlooper, the leading Democratic candidate, faced a brief spate of criticism in October for hesitating on a question about impeachment at a Montrose forum, but has since made clear his support.

“I think the Senate needs to have a fair trial and that the president’s aides have to be allowed to testify,” Hickenlooper said in a statement Dec. 18. “The president has so far silenced key witnesses and withheld key documents, and that’s wrong. The American people deserve the full truth.”

Democratic candidate Andrew Romanoff said Dec. 11, “If I were in the Senate, I would vote to convict Donald Trump and remove him from office.” Another Democratic challenger, Trish Zornio, has said Gardner “isn’t serious about the oath he took” to the Constitution. Democratic candidate Lorena Garcia predicted Dec. 18 that “the GOP-led Senate will put their party over the preservation of our Constitution.”

Because he faces a tough re-election fight, there has been significant speculation among national pundits that Gardner will be a swing vote during the impeachment trial, but the senator himself has never suggested that. Instead, he has consistently criticized the impeachment of Trump and consistently voted in accordance with the president’s policy interests.

“One thing to remember: Gardner is the most talented politician on the Republican team, so if anyone’s going to thread those needles, it will be him, but it’s still a really difficult task,” Saunders said.

Colorado’s other senator, Denver Democrat and presidential contender Michael Bennet, has been more outspoken. A spokeswoman said Bennet believes everyone who had knowledge of Trump’s decision to withhold aid from Ukraine should testify, including former national security adviser John Bolton, acting chief of staff Mick Mulvaney, budget official Michael Duffey, and White House aide Robert Blair.

“The recent reporting and evidence that has come to light even after the House voted to impeach only reinforces Michael’s concerns that the Senate needs to hear from witnesses and have access to all relevant documents,” said the senator’s spokeswoman, Courtney Gidner.

The Senate has a responsibility to hold a thorough trial, Gidner said, and McConnell’s decision to work closely with the Trump administration jeopardizes that.