WASHINGTON - The impeachment trial of President Donald Trump, expected to open in the Senate on Thursday, is shining an intense spotlight on a handful of Senate Republicans who hold the power to decide how the trial will proceed.

For weeks, Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah and Lamar Alexander of Tennessee have been questioned about the twists and turns of the House impeachment proceedings, but they have largely declined to comment, waiting until the issue surfaced in their chamber.

But they will now become central characters in the highly anticipated impeachment trial of Trump, where many of the partisan lines have already been drawn and Democrats have vowed to exert pressure on the group to break with their party on key questions such as obtaining documents and calling witnesses.

Whether Trump's trial uncovers new information through fresh witnesses and documents or ultimately becomes a largely preordained exercise will likely come down to these Republican senators - who, at least publicly, are shrugging off the scrutiny.

"People can express their own view if they'd like," Romney said Tuesday. "I intend to be as impartial as the oath requires."

Despite their role as potential swing GOP votes in a narrowly divided Senate, they have yet to defect in any significant fashion from party leaders, who in turn have been willing to accommodate the group's requests as Republicans finalize a measure that will set the parameters of the trial. After a formal start set tentatively for Thursday, the bulk of the trial is set to begin in earnest next week.

In one nod to the group, there is expected to be a provision guaranteeing a vote on whether the Senate could consider subpoenaing witnesses, according to two GOP officials familiar with the matter, who spoke on the condition of anonymity because the resolution has not been made public. Collins had indicated last week that she wanted to ensure that senators will get to vote on the ability to call witnesses.

Senate Majority Leader Mitch McConnell, R-Ky., and a number of other senators, including members of his leadership team, has made it clear that he would prefer no witnesses be called during the trial but has not foreclosed the possibility.

Both Democrats and even some Republicans, however, have vowed to force votes on witnesses - potentially sending the Senate into chaos. McConnell noted Tuesday that both parties would get a say on witnesses, telling reporters: "I can't imagine that only the witnesses that our Democratic colleagues would want to call would be called."

GOP leaders are confident that once voting begins to set the scope of the trial - called an organizing resolution - that no Republicans will defect, with the moderates placated by a guaranteed decision on witnesses later.

That calculus could change once the Senate goes through the grind of opening arguments and a litany of questions, and if key GOP senators become dissatisfied that they hadn't gotten enough information from the trial proceedings. Though the likes of Romney, Collins, Murkowski and Alexander have been the most closely watched, other endangered Republicans on the ballot this year - such as Cory Gardner of Colorado and Martha McSally of Arizona - are also being scrutinized.

For weeks, Democrats have pushed for four current and former administration officials to testify, including acting White House chief of staff Mick Mulvaney and former national security adviser John Bolton.

Bolton, who could shed more light on whether Trump withheld military aid and a White House visit from Ukraine to force its president to investigate his political rivals, said last week that he would be willing to testify before the Senate if subpoenaed.

Romney said this week that he presumes he will vote in favor of hearing from Bolton, though he added that his view could change depending on what he hears from the trial. He also said Tuesday that he doesn't "plan to put a list together" of desired witnesses. Last week, Murkowski said she would be "curious" as to what Bolton would have to say, but she had not made a commitment on whether she wants to hear from the former White House official.

"I won't know until we get there," Murkowski said this week. "I need to hear first from both sides. I'll only be able to formulate my questions [while listening] to the questions and responses from members. We'll all have the opportunity to weigh in. That's what we're trying to do is make sure that we all have a guaranteed opportunity [to weigh in]."

Others, such as Collins and Alexander, have declined to specify which witnesses, if any, they would like to hear from and likely will not until after the first phase of the trial is over.

"We have a constitutional responsibility here. Just because the House was a circus doesn't mean the Senate needs to be," Alexander said Tuesday. "So we should hear the case, not dismiss it. We should hear the arguments, we should ask our questions, and then we should vote on whether we need additional evidence. And I think that's a fair and impartial way to go about it."

White House legislative affairs director Eric Ueland on Tuesday declined to say whether the administration was paying special attention to the requests of the four senators and other potential swing votes, suggesting that officials were watching all Republicans as the trial progresses.

"We are very cognizant of all 53 members and their priorities, needs, objectives," he said. "And we are continuing to engage in robust work and robust conversations and partnership with a lot of Senate and House GOP members up here, as we have for the last several months."

Collins, who is up for reelection this year, is poised to join three other Republicans in voting for a resolution curbing Trump's military authority in Iran.

The impeachment trial is also reminiscent of other high-stakes votes in which Collins and Murkowski, in particular, have wielded considerable power.

In fall 2018, Collins and Murkowski were the final undecided swing votes during the confirmation battle over then-Supreme Court nominee Brett Kavanaugh, who was accused of sexual misconduct during his teenage and young adult years. (He denied the allegations.) Collins ultimately fell in line, while Murkowski - in a dramatic 11th-hour announcement - said she could not support the nominee.

Kavanaugh was confirmed on a near party-line vote of 50 to 48 on Oct. 6. That day, Trump told The Washington Post that Murkowski would "never recover from this."

In July 2017, both Collins and Murkowski defied their party on health care, dooming Republican efforts to fully undo the Affordable Care Act. They were joined by Sen. John McCain, R-Ariz.; together, the three sunk a measure that would have killed key funding and protections provided by the law.

Five months earlier, Collins and Murkowski voted in concert against Trump's education secretary nominee, Betsy DeVos, whom they deemed unqualified. Vice President Mike Pence was summoned to the Capitol to break a 50-50 tie on the nomination, a first.

The two senators have already shown a willingness to buck their party on impeachment, refusing in October to sign on to a resolution that condemned the House inquiry. Romney joined them in declining to support the measure.

Romney has maintained his own independent streak since he joined the Senate early last year, criticizing Trump on trade, opposing a judicial nominee who called former president Barack Obama an "un-American impostor" and leading the charge against Herman Cain as a potential member of the Federal Reserve Board.

As of mid-2019, Romney had voted against Trump more than any other Senate Republican, while Collins holds that distinction now, according to a ranking by FiveThirtyEight. More recently, Romney has been one of the loudest Republican critics of Trump calling on Ukraine and China to investigate former vice president Joe Biden, the issue at the heart of the impeachment inquiry.

Meanwhile, Alexander, a close McConnell ally, is a veteran of the chamber and one of its remaining institutionalists who also has bucked the president, including to reject Trump's emergency declaration issued last year to redirect federal funding for a border barrier. He is retiring after this term.

"Any four people could be powerful," said Sen. Lindsey Graham, R-S.C., an ardent Trump ally. "I respect them all."

This article was written by Seung Min Kim and Elise Viebeck, reporters for The Washington Post.