As Maine voters head to the polls Tuesday, they'll decide whether the U.S. Senate will keep one of its most controversial members.

Sen. Susan Collins, R-Maine, is currently in a tough re-election battle after more than two decades in Washington. A perceived moderate, she's made a series of decisions that have infuriated both sides of the political spectrum.

Here are five things to know about her:

Controvesially supported Justice Kavanaugh's confirmation 

One of the main drivers of progressive opposition to Collins is the fact that she voted in favor of Justice Brett Kavanaugh's confirmation to the Supreme Court in 2018. She was one of the few Republicans who observers thought might buck the party line. Her decision helped place another conservative justice on the Supreme Court at a time when progressives accused Republicans of ignoring rape allegations against Kavanaugh.

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According to "Justice on Trial: The Kavanaugh Confirmation and the Future of the Supreme Court," an unnamed Republican senator wanted her to ditch Kavanaugh after his accuser, Christine Blasey Ford, delivered emotional testimony to the Senate Judiciary Committee. The senator reportedly wanted Collins to join him in asking the White House to withdraw the Kavanaugh nomination in exchange for a pledge to approve President Trump's replacement nominee.

Perceived as a moderate

Collins has long been perceived as a moderate Republican on whom the party couldn't rely for certain votes. Conservatives have also derided her as a "RINO" or "Republican in Name Only."

In addition to Kavanaugh's confirmation, however, Collins voted to pass President Trump's tax package in 2017. Despite the criticism over the two votes, Collins reportedly said she was "proud" of both actions.

“What I’ll never know, and never understand is why anyone would think that I would cave in to threats and do the wrong thing instead of the right thing. They don’t know me,” Collins reportedly said at an event earlier this year.

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In 2019, GovTrack ranked Collins as the third most "politically left" Republican senator -- surpassed only by Sens. Lisa Murkowski, R-Alaska, and Richard Shelby, R-Ala.

Critical of Trump but acquitted him of impeachment charges

Collins has been a vocal critic of President Trump's conduct in office, including when he told the "Squad" of Democratic congresswomen to "go back and help fix the totally broken and crime-infested places from which they came." Those comments were "way over the line," she said.

Collins also voted to acquit Trump after he was impeached for, in part, a phone call in which he mentioned corruption allegations surrounding former Vice President Joe Biden in a call with Ukrainian president Volodymyr Zelensky. While Trump defended the call, Collins said it was "far from a perfect call" but that his conduct didn't rise to the level that would warrant his removal from office.

"I do not believe that the House has met its burden of showing that the president’s conduct – however flawed – warrants the extreme step of immediate removal from office,"  Collins said. However, both she and Sen. Mitt Romney, R-Utah, broke with their party to vote in favor of allowing new witnesses in the Senate's trial. 

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She's running in a northeastern, Democratic state

Even without friendly fire, Collins is already facing tough odds as she's running in a state that has repeatedly supported Democratic presidential candidates.

Although three of the six New England states — Massachusetts, New Hampshire and Vermont — have moderate Republican governors, 10 of the region’s 12 senators are Democrats, while Maine’s junior senator, Angus King, is an independent who caucuses with Democrats.

According to RealClearPolitics, polling has consistently shown Collins' Democratic opponent, Sara Gideon garnering more support.

Voted against Barrett nomination, opposes overturning Roe v. Wade

Collins faced intense pushback on another Supreme Court nominee -- now-Justice Amy Coney Barrett -- just before Election Day. The Maine senator voted against Barrett's confimation and had announced she opposed Trump's nominating someone so close to the election. 

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Barrett's confirmation last month seemed to solidify the conservative majority some activists have sought in order to strike down longstanding abortion precedent. While abortion has always been a testy issue in American politics, recent court and legislative battles have raised the prospect that significant changes could be made to Roe v. Wade, which blocked some state prohibitions on the procedure. Democratic candidates, in particular, have vowed to codify the landmark decision into law.

Collins previously expressed her opposition to judges who would overturn Roe. 

“A candidate for this important position who would overturn Roe v. Wade would not be acceptable to me, because that would indicate an activist agenda that I don’t want to see a judge have,” she said.

Fox News' Fred Lucas and Gregg Re contributed to this report.