She has called the Trump administration “constant chaos’’ and says reports that the president shared classified information with Russian government officials are “very troubling.’’

From her perch on the Senate Intelligence Committee this week, she called for fired FBI director James Comey to furnish memos he wrote about conversations with Trump about his campaign’s alleged dealings with Russia.

A lockstep Democrat looking to run up the score as the White House operates under siege? In fact, it’s a 20-year incumbent senator from Trump’s own party.

Less than four months into Donald Trump’s presidency, Republican Susan Collins of Maine has become a pivotal player as the administration grows increasingly embroiled in questions of potentially criminal actions and the GOP struggles to respond.

She is the most senior female senator in the Republican caucus and sits on the Intelligence Committee, which has asked for a briefing on the exchange last week between Trump and senior Russian officials. She was also one of a small, but growing, number of Senate Republicans who called for an independent investigation before Wednesday, when the Justice Department appointed special counsel Robert Mueller.

Collins said her committee would continue its own “broader’’ investigation, which will focus on policy issues — such as potential new sanctions against Russia — while Mueller’s focus is primarily criminal.

“I think our committee, the Intelligence Committee, already has sufficient evidence that we’re pretty close to being able to find that there was Russian interference in our election last fall,’’ Collins said in a phone interview Thursday.

If former national security adviser Michael Flynn does not comply with the committee’s subpoena, Collins told the Globe that the panel could exercise the option of holding him in contempt.

“Usually what you do in these investigations is you ratchet it up,’’ she said.

It’s an uncommon position even for a longtime politician: partaking in a global-stakes investigation of a president from one’s own party.

Maine state Republican Party officials acknowledged that their senior senator “finds herself standing in a unique and difficult situation in Washington.’’

Lisa Camooso Miller, a Washington public affairs consultant and former Republican National Committee communications director, noted that Collins, who declared that she would not vote for Trump in the November election, is now singularly positioned to speak up. “She doesn’t have to hide behind the political filter that many, many others would have to,’’ she said. “She doesn’t have to hide behind where she was during the election.’’

Collins’s frequent breaks with Trump — she was the only Senate Republican to vote against Scott Pruitt’s nomination to lead the Environmental Protection Agency — are not an unalloyed surprise. As the only Senate Republican from New England, she watched Democrat Hillary Clinton carry her state last year by nearly 3 points.

Centrists in both parties have praised the dwindling number of members of Congress who occupy the political middle, citing a long-ago past, real or imagined, when Washington functioned with less acrimony. Maine itself has long produced a cast of more moderate senators who are not hyperpartisan.

“I’m proud of the approach that the Senate Intel Committee has taken. Unlike the House, the Senate approach has been doggedly bipartisan,’’ Collins said Thursday.

But if allegations against Trump continue gathering momentum, Collins’s self-described moderate stance could face the test of the nastiest battles in Washington: federal probes and potential impeachment. Longtime Washington figures are now openly using “the I-word’’ with regards to Trump.

“The key to whether there is a serious investigation on Capitol Hill or not largely resides with what, if anything, Senate majority leader McConnell is going to do,’’ said James Manley, a Democratic strategist who worked for former majority leader Harry Reid of Nevada, the late senator Edward M. Kennedy of Massachusetts, and former majority leader George Mitchell of Maine.

“Forget about the House,’’ said Manley. “And the only way McConnell is going to allow an aggressive investigation is if he hears enough from his caucus they’ve had enough, which includes Senator Collins.’’

Others question whether Collins has been tough enough.

“She could be more forceful. Her statements are rather, shall we say, minimalist in taking a strong position,’’ said Janet Martin, a professor of government at Bowdoin College. “And she’ll pretty much wait until she knows it would be safe to take a position against the president.’’

Collins votes in alignment with Trump nearly 88 percent of the time, according to a tracker maintained by the website FiveThirtyEight, signing off on his Supreme Court nominee Neil Gorsuch and, last week, initially voicing support for his firing of Comey.

But, in January, she joined with a handful of other GOP senators opposing a temporary ban on immigrants from seven majority-Muslim countries and in March opposed an effort to strip federal funds from health centers that perform abortions. Last week, she voted against an administration-backed resolution nullifying a rule requiring energy companies to reduce pollution.

She also voted against the president’s choice for education secretary.

And last August she took to the Washington Post op-ed page to announce she would not be voting for Trump.

With mounting questions about the administration, other Republicans have been edging away from the president, particularly after explosive reports Tuesday that Comey had written a memo based on a meeting with Trump in which he said the president asked him to drop an investigation into Flynn’s dealings with Russia.

Maine’s other senator, Angus King, an independent who also sits on the intelligence committee, told CNN late Tuesday night, that he “reluctantly’’ thought that, if Comey’s reported allegations are true, the country was coming closer to impeachment.

If things progress that far, Collins would likely play a key role. At the same time, she is postponing a decision until “early this summer’’ on whether to run for governor next year, where her actions during the investigations would surely become an issue.

“The question is whether Senator Collins and others are going to push Senator McConnell to get something done,’’ said Manley.

Senator Elizabeth Warren, the Massachusetts Democrat and a frequent Trump critic, said she came to know Collins through their work on aging and medical research funding, calling her “focused and deeply informed.’’

Asked whether she was surprised at Collins’s vocal criticism of Trump, Warren laughed, saying, “No. That’s Susan. She’s going to row her own boat here.’’

Jim O’Sullivan can be reached at jim.osullivan@globe.com. Follow him on Twitter at @JOSreports.