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."
U.S. Sen. Lamar Alexander through the years
U.S. Sen. Lamar Alexander through the years
WASHINGTON, DC - JANUARY 21: US Republican Senator Lamar Alexander (R) from Tennessee addresses US President Barack Obama during the Inaugural Luncheon in Statuary Hall on Inauguration day at the U.S. Capitol building January 21, 2013 in Washington D.C. President Barack Obama and Vice President Joe Biden were ceremonially sworn in for their second term today. (Photo by Matt Cavanaugh-Pool/Getty Images)
UNITED STATES - JULY 9: Sen. Lamar Alexander, R-Tenn., speaks to the media about student loans as Senate Minority Leader Mitch McConnell, R-Ky., listens after the Senate policy luncheons on Tuesday, July 9, 2013. (Photo By Bill Clark/CQ Roll Call)
UNITED STATES - Sept 17: Senators Lamar Alexander, R-TN., and Marco Rubio, R-FL., make their way to the Senate policy luncheons through the Senate subway in the U.S. Capitol on September 17, 2013. (Photo By Douglas Graham/CQ Roll Call)
WASHINGTON, DC - SEPTEMBER 26: Senator Lamar Alexander (R-TN), right, meets with super model Niki Taylor, left, and NASCAR driver Denny Hamlin to discuss support for maternal and infant health issues, including Preemie Act and the Newborn Screening Save Lives Acton on September 26, 2013 in Washington, DC. (Photo by Paul Morigi/WireImage)
WASHINGTON, DC - OCTOBER 8: Senators Lamar Alexander (R-TN) and Tom Coburn (R-OK) walk over to the House side on the eighth day of a government shutdown with no Congressional agreement in sight, on Capitol Hill Tuesday October 8, 2013. (Photo by Melina Mara/The Washington Post via Getty Images)
US President Barack Obama steps off Air Force One upon arrival at McGhee Tyson Airport on January 9, 2015 in Alcoa, Tennesee. Obama is in Tennessee to speak on new proposals for higher education accessibility. With Obama are Senator Lamar Alexander (C), R-TN, Senator Bob Corker, R-TN, and Rep. John Duncan, R-TN. AFP PHOTO/MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP via Getty Images)
UNITED STATES - JULY 26: Sen. Lamar Alexander, R-Tenn., speaks to reporters following votes during the Sunday session in the Senate on July 26, 2015. (Photo By Bill Clark/CQ Roll Call)
WASHINGTON, DC - MAY 23: Senator Lamar Alexander (R-TENN) speaks after he received an during a reception to honor him with the SupportMusic Champion Award at Nelson Mullins on May 23, 2016 in Washington DC. (Photo by Kris Connor/Getty Images for NAMM)
UNITED STATES - JANUARY 17: Chairman Sen. Lamar Alexander, R-Tenn., waits for Secretary of Education nominee Betsy DeVos to arrive testify during her confirmation hearing in the Senate Health, Education, Labor and Pensions Committee on Tuesday, Jan. 17, 2017. (Photo By Bill Clark/CQ Roll Call)
UNITED STATES - SEPTEMBER 14: Sen. Lamar Alexander, R-Tenn., speaks to reporters as he boards the Senate subway in the Capitol on Thursday, Sept. 14, 2017. (Photo By Bill Clark/CQ Roll Call)
WASHINGTON, DC - NOVEMBER 29: (L-R) Senator Al Franken (D-MN) and Chairman Lamar Alexander (R-TN) attends full committee hearing on the nomination of Alex Michael Azar II to be Health and Human Services Secretary on Capital Hill on November 29, 2017 in Washington, DC. (Photo by Tasos Katopodis/Getty Images)
UNITED STATES - OCTOBER 18: Sen. Lamar Alexander, R-Tenn., speaks with reporters in the Senate subway in the Capitol on Wednesday, Oct. 18, 2017. (Photo By Bill Clark/CQ Roll Call)
WASHINGTON, DC - DECEMBER 19: (L-R) U.S. Sen. Susan Collins (R-ME), Sen. Lindsey Graham (R-SC) and Sen. Lamar Alexander (R-TN) arrive for a Senate Republican policy luncheon at the Capitol December 19, 2017 in Washington, DC. The House has passed the tax overhaul bill and the Senate is expected to vote on the bill soon. (Photo by Alex Wong/Getty Images)
China's Premier Li Keqiang (R) shakes hands with US Senator Lamar Alexander of Tennessee during a meeting with a group of U.S. Republican senators and Congress members at the Zhongnanhai leadership compound in Beijing on November 1, 2018. - Chinese Premier Li Keqiang told a visiting delegation of seven US Republican lawmakers on November 1 that he hoped their two countries can meet "halfway" amid friction over trade, security and other issues. (Photo by JASON LEE / POOL / AFP) (Photo credit should read JASON LEE/AFP via Getty Images)
UNITED STATES - JUNE 18: Chairman Sen. Lamar Alexander, R-Tenn., takes his seat for the Senate Health, Education, Labor and Pensions Committee hearing on the "Lower Health Care Costs Act of 2019on Tuesday, June 18, 2019. (Photo By Bill Clark/CQ Roll Call)
UNITED STATES - MARCH 26: Sen. Lamar Alexander, R-Tenn., exits the Senators Only elevator in the Dirksen Senate Office Building on Wednesday, March 27, 2019. (Photo By Bill Clark/CQ Roll Call)
UNITED STATES - FEBRUARY 2: Sen. Ted Cruz, R-Texas, left, and Sen. Lamar Alexander, R-Tenn., talk as they leave the Senate Republicans' lunch in the Capitol on Tuesday, Feb. 2, 2019. (Photo By Bill Clark/CQ Roll Call)
UNITED STATES - DECEMBER 11: Sen. Lamar Alexander, R-Tenn., arrives in the Capitol for a vote on Wednesday, Dec. 11, 2019. (Photo By Bill Clark/CQ-Roll Call, Inc via Getty Images)
WASHINGTON, DC - JANUARY 21: Sen. Lamar Alexander (R-TN) heads toward the Senate Chamber before the start of President Donald Trump's impeachment trial at the U.S. Capitol January 21, 2020 in Washington, DC. Senators will vote Tuesday on the rules for the impeachment trial, which is expected to last three to five weeks. (Photo by Chip Somodevilla/Getty Images)
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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.
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.