As President Trump's impeachment trial approaches, Sen. Susan Collins, R-Maine, is under close scrutiny from Democrats and her fellow Republicans ahead of a vote that could once again test her reputation for centrism and independence.

The spotlight on Collins has come a bit earlier than expected.

In the narrowly divided Senate, just four Republicans could force Majority Leader Mitch McConnell to reconsider his vow for a speedy acquittal and "total coordination" with the Trump White House.

Collins is among the potential swing votes, and she already faces a difficult reelection in a state deeply divided over the president. Hillary Clinton won Maine in 2016, but Donald Trump made history by snagging an electoral vote despite losing statewide by winning big in the state's vast and rural 2nd Congressional District.

Since August, Republican and Democratic groups have been running ads designed to influence Collins' reelection, although she didn't announce her bid until mid-December — the same day the House of Representatives voted to impeach Trump.

It was curious timing. Collins had already seen her once sky-high favorability drop because of controversial votes she has taken during the Trump presidency.

Collins, who is seeking a fifth term, has generally said little about impeachment, citing her role as a juror, a position her Democratic challengers say is a cop-out.

Now, Trump's opponents want her to use her leverage to ensure a fair and thorough trial. The group Republicans for the Rule of Law has joined the Democrats' pressure campaign to get Collins to push for documents and witnesses whom the White House has so far blocked from testifying. Collins has been characteristically quiet about what she'll do, but in an interview with Maine Public Radio, she said there should be bipartisan agreement on how the trial goes, just as there was during President Bill Clinton's impeachment trial in 1999.

At the same time, Collins acknowledged that this kind of agreement seems unlikely today.

"One hundred to zero," said Collins, referencing the 1999 Senate vote outlining the framework of the impeachment trial. "I can't imagine anything like that happening today, regrettably."

She also echoed recent criticism from her friend and close ally Alaska Republican Sen. Lisa Murkowski, who told a local TV station in Anchorage that she was "disturbed" by McConnell's vow to coordinate with the White House.

Collins called McConnell's declaration inappropriate but also chastised Democrats like Massachusetts Sen. Elizabeth Warren for saying she's ready to convict the president.

"There are senators on both sides of the aisle who, to me, are not giving the appearance of — and the reality of — judging this in an impartial way," she told Maine Public Radio this week.

Like Murkowski, Collins made headlines for her criticism of McConnell. But both were critical of Democrats in the House, saying they rushed the impeachment vote and did not ask the courts to compel Trump aides to testify.

Collins also said House Speaker Nancy Pelosi should transmit the articles of impeachment to the Senate instead of holding on to them.

"So it seems an odd way to operate," Collins said.

Unlike many Senate Republicans, Collins says that she's open to witnesses in the Senate trial but also that it's too early to say which ones. That drew a rebuke from the Maine Democratic Party for not using her clout to ensure key witness testimony and documents.

"Senator Collins often touts her position of power within the Republican caucus and clearly has a close relationship with Mitch McConnell, and if a fair and impartial impeachment trial is really her priority, it's time for her to act to ensure that is what takes place," Maine Democratic Party Executive Director Lisa Roberts said in a statement released Thursday.

In her Maine Public Radio interview, Collins stopped short of saying she'll publicly lead the charge to call for witnesses and testimony. She said she has discussed the matter, as well as her desire for a bipartisan agreement, with McConnell.

Her critics have noted that she was more forceful about her desire for more information during the Clinton impeachment.

"I need more evidence. I need witnesses and further evidence to guide me to the right destination, to get to the truth," Collins said at the time.

The president's supporters are also keeping a close eye on Collins, who declared in 2016 that Trump was unfit for office.

If she breaks ranks on impeachment, Collins could draw a primary challenge. The deadline for the primary is in mid-March. The Maine Republican Party has said it's united behind Collins.

Nevertheless, Trump is also paying attention.

Last week he retweeted an endorsement of Collins' reelection bid to his 68 million followers.

It marked the first time Trump has mentioned Collins in a tweet since he became president.