Sen. Susan Collins Susan Margaret CollinsSenate GOP set to vote on Trump's Supreme Court pick before election Democratic senator to party: 'A little message discipline wouldn't kill us' Poll: 57 percent of Americans think next president, Senate should fill Ginsburg vacancy MORE (R-Maine) is emerging as the most important vote in the Senate battle over President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE’s impeachment trial, as Democrats regard her support as key to getting the additional witnesses and documents they need to build their case.

Collins, who has played a pivotal role in the biggest Senate debates of the Trump era, finds herself once again in the spotlight as one of the Republicans considered most likely to side with Democrats.

“She’s an important player,” said Senate Democratic Whip Dick Durbin Richard (Dick) Joseph DurbinHawley warns Schumer to steer clear of Catholic-based criticisms of Barrett Two Judiciary Democrats say they will not meet with Trump's Supreme Court pick Sunday shows preview: Lawmakers prepare for SCOTUS confirmation hearings before election MORE (D-Ill.), who noted that while every Senate vote is important, key trial questions will be decided by a handful of lawmakers.

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“Four Republicans can bring this back to a bipartisan position,” he said.

Collins cast pivotal votes to defeat legislation to repeal ObamaCare in 2017 and to confirm Supreme Court Justice Brett Kavanaugh Brett Michael KavanaughSenate GOP set to vote on Trump's Supreme Court pick before election Conservative groups unleash ad blitz in support of Barrett's nomination Two Judiciary Democrats say they will not meet with Trump's Supreme Court pick MORE in 2018.

Partisan lines have become so entrenched in the Senate that only a handful of senators are considered potential defectors on contentious votes.

Sen. Debbie Stabenow Deborah (Debbie) Ann StabenowSunday shows preview: Lawmakers prepare for SCOTUS confirmation hearings before election OVERNIGHT ENERGY: Trump rollbacks could add 1.8 billion tons of greenhouse gas emissions over 15 years: analysis | Intensifying natural disasters do little to move needle on climate efforts | Experts warn wildfire smoke could worsen COVID-19 GAO report finds brokers offered false info on coverage for pre-existing conditions MORE (Mich.), chairwoman of the Senate Democratic Policy and Communications Committee, said there’s “no question” that Collins will be a pivotal vote. She cited Sen. Mitt Romney Willard (Mitt) Mitt RomneyCrenshaw looms large as Democrats look to flip Texas House seat The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-Utah) as another potential swing vote.

But Stabenow said fellow senators don’t know what to expect from Collins, who has kept her cards close to the vest.

“That is one of the big questions at this point,” she said. “We’re getting no indication at all.”

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A Democratic senator who requested anonymity said the Democratic caucus is most focused on Collins and Sen. Lisa Murkowski Lisa Ann MurkowskiSenate GOP set to vote on Trump's Supreme Court pick before election Supreme Court fight pushes Senate toward brink Democratic senator to party: 'A little message discipline wouldn't kill us' MORE (R-Alaska).

“Susan Collins and Lisa are key. Everyone is watching them because Susan is up for reelection and we all know what the party did to Lisa, so she has an interest in being independent,” the senator said.

In 2010, Murkowski lost the Alaska GOP Senate primary but won the general election as a write-in candidate.

When asked by The Hill whether she’s experiencing a sense of déjà vu about being the center of attention, Collins replied, “That’s not what I’m seeking to be. I’m just trying to do my job.”

A Senate source familiar with Collins’s thinking said the senator will likely play a significantly different role than she did during the Kavanaugh debate when she joined with other moderates such as Murkowski, then-Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (R-Ariz.), then-Sen. Heidi Heitkamp Mary (Heidi) Kathryn HeitkampHarris faces pivotal moment with Supreme Court battle Centrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents MORE (D-N.D.) and Sen. Joe Manchin Joseph (Joe) ManchinTrump taps Amy Coney Barrett for Supreme Court, setting up confirmation sprint Sunday shows preview: Lawmakers prepare for SCOTUS confirmation hearings before election Supreme Court fight pushes Senate toward brink MORE (D-W.Va.) to call for an FBI investigation into sexual assault allegations confronting Kavanaugh before his nomination went forward.

Collins is not expected to once again form a rump group with fellow Senate moderates to demand more information relevant to the articles of impeachment against Trump, the source said.

Moderate Democrats such as Sens. Christopher Coons Christopher (Chris) Andrew CoonsTwo Judiciary Democrats say they will not meet with Trump's Supreme Court pick Sunday shows preview: Lawmakers prepare for SCOTUS confirmation hearings before election Trump's push for win with Sudan amps up pressure on Congress MORE (Del.) and Manchin confirmed that there has been little in the way of formal discussions among the centrists about what the rules for the Senate trial should be.

Another difference this time: Collins will play the role of Senate arbiter while also facing a potentially competitive primary and general election in the same year.

“She would definitely have a primary challenger if she did break ranks with the Republicans,” said Janet Martin, a professor of government at Bowdoin College in Maine.

Martin said Maine has a strong Tea Party faction, particularly in the state’s rural 2nd Congressional District, that could mobilize against Collins if she votes with Democrats on crucial procedural votes.

Collins witnessed the strength of Maine’s Tea Party activists in 2010 when they took control of the state Republican Party convention to adopt an aggressive platform that called for radical changes such as sealing the nation’s borders and eliminating the Department of Education and the Federal Reserve.

Trump visited Maine four times during the 2016 campaign in hopes of winning one of Maine’s electoral votes. Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonThe Memo: Trump furor stokes fears of unrest Bloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close MORE ended up narrowly carrying the state by 2.5 percentage points but won only three of its four electors.

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Maine’s Senate Republican primary won’t be held until June 9, giving a potential opponent plenty of time to wage a campaign based on Collins's impeachment votes.

Another important consideration for Collins is that she’s in line to become the next Republican chairwoman of the powerful Senate Appropriations Committee, as Sen. Richard Shelby Richard Craig ShelbyThe Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Senate to push funding bill vote up against shutdown deadline Senate GOP eyes early exit MORE (R-Ala.) will eventually reach the end of his panel term limit and Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderPelosi urges early voting to counter GOP's high court gambit: 'There has to be a price to pay' Graham: GOP has votes to confirm Trump's Supreme Court nominee before the election The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Tenn.) will retire at the end of next year.

Collins hinted during an interview with Newsradio WGAN in Maine that she has the Appropriations gavel in mind.

“But for my position on the Appropriations Committee, a committee that I’d be in line to chair if I’m reelected, I would not be able to accomplish those goals for the state of Maine. It makes a real difference,” she said, citing her work on the military's so-called widow's tax, community development funding and a new ship for the Maine Maritime Academy.

Who becomes the next Appropriations head is largely determined by seniority, but the selection must be ratified by a vote of the entire Senate GOP conference, and there’s a chance that Trump would oppose her ascension if she voted to convict him on articles of impeachment or dealt him a significant procedural setback on witnesses or documents.

Republican senators are confident Collins will stick with them on their initial demand that the question of whether to subpoena additional witnesses and documents should wait until after the impeachment managers and Trump’s defense team present their arguments.

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“It seems near unanimous, if it’s not unanimous,” said Sen. Kevin Cramer Kevin John CramerNetflix distances from author's comments about Muslim Uyghurs but defends project Abortion stirs GOP tensions in Supreme Court fight Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-N.D.) of the view that the opening phase of Trump’s Senate trial should follow the precedent set by former President Clinton’s 1999 trial.

Yet Democrats think they have a chance of picking up Collins on key impeachment votes, speculating she might be trying to burnish her centrist credentials after feeling a public backlash for voting to confirm Kavanaugh last year.

“I think the Kavanaugh vote, in talking to some people who are more familiar with her staff, is something that has been distressful because of the reaction that has come about,” Martin said.

Democrats and allied liberal groups are making her Kavanaugh vote one of their central talking points ahead of the Maine Senate race.

Emily’s List, a Democratic-allied group, circulated a fundraising email Friday highlighting Collins’s statement that since the bitter Supreme Court fight of 2018, she doesn’t regret her vote “in the least.”