His political hero is former Sen. Howard Baker, the Republican Tennessee lawmaker remembered for his impartiality during the Watergate impeachment hearings.

Now, all eyes are on Sen. Lamar Alexander, R-Tenn., as he could be a pivotal vote on whether there are witnesses in President Donald Trump's Senate impeachment trial.

Alexander hasn't tipped his hand and no one is totally sure of which way he'll go.

'You can't predict him," Tom Ingram, Alexander's former chief of staff, told NBC News. "He will hold his counsel, make his own decision and you won’t be sure of it until he makes it known in due course."

Alexander, who is retiring at the end of this term and has a history of working with Democrats on major issues, has been zeroed in on along with GOP Sens. Mitt Romney, Susan Collins and Lisa Murkowski as the top targets for Democrats hoping to have witness testimony and documentary evidence at the Senate trial. Four Republicans will need to vote alongside all Democrats in order for new witness testimony to be admitted.

While the latter three have all expressed, with varying intensity, an interest in having additional testimony in the trial, particularly after the reported revelations in former national security adviser John Bolton's upcoming book, Alexander has not given a strong hint as to his position, saying he won't make a determination until opening arguments and senators' questions are complete, which could be Tuesday.

"After we've heard all the arguments, after we heard the questions and the answers to the questions and we've studied the record then we'll have that vote" about whether to call witnesses, Alexander said Monday. "And at that time, I'll make a decision about whether I think they need additional evidence."

Still, Alexander's potentially huge role in the impeachment trial has garnered plenty of attention.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Richard Clinton, political science professor emeritus at Oregon State and fraternity brother of Alexander's at Vanderbilt, authored an open letter to the senator calling on him to have the "courage" to vote for witnesses.

Mark Braden, who served as Alexander's campaign manager in 2014, told NBC News Alexander "is trusted by Tennesseans to do what he believes is right regardless of whoever might be shouting the loudest on Twitter or cable news."

While he has called the House investigation a "circus," Alexander pushed Senate Majority Leader Mitch McConnell, R-Ky., to allow for a vote on witnesses in the resolution on the trial rules. And should he vote in favor of new evidence, that may be enough for more than just four Republicans to do the same.

"If it's Lamar Alexander, widely respected in the caucus, retiring, an institutionalist who's been a part of a lot of negotiations over the years...If he were to break, I think we could get maybe double that," former Sen. Claire McCaskill, D-Mo., an NBC News/MSNBC analyst, said last week of whether Democrats can land four additional votes.

In Tennessee, Republicans and Republican-leaning voters were split on whether Alexander should vote to call witnesses.

"Any trial I have either served on a jury at, upon or whatever, both sides call witnesses," Henry Howard told NBC News in Murfreesboro, Tenn.

"If you are on trial, if I was on trial, my attorney would be calling witnesses," he added. "The prosecution would be calling witnesses. This, and impeachment is, according to my definition, a trial."

Teresa Fly felt similarly about witnesses, telling NBC News that "if somebody hit your car and caused an accident, especially with injuries, and there were three people who witnessed that, would you not really want them to be able to say what they saw?"

"How could you have justice and not have people who know things say what they know under oath?" she asked. "And of course, we all know that a lot of people lie. Okay, but not everybody lies, and some people will step up to the mark when it's important and do the right thing."

She added, "If he wants to think about his legacy to the people, I think he really needs to give some special consideration to his vote."

Alexander, 79, is a renowned political figure in the state. He began his political career as a legislative assistant to Baker, was elected twice as governor of Tennessee, served as President George H.W. Bush's education secretary and twice ran for president before serving three terms in the Senate, where he now chairs the Senate Health, Education, Labor and Pensions Committee.

Ingram, the former chief of staff, said Alexander would approach the vote "based solely and exclusively on what he believes is right."

"Worrying about what he’s going to do may be natural for some, but it will not make any difference to him," Ingram added. "He will make his own decision regardless of who is the president, his party, what his colleagues in the Senate may say, regardless of whether he is or is not seeking reelection, or how his legacy may be viewed, and he will have a clear conscience doing it."

Cal Perry reported from Murfreesboro, Tenn., and Allan Smith from N.Y.