Sen. Susan Collins, R-Maine, is facing a tough reelection bid Tuesday as she faces down a prominent state Democrat, Sara Gideon, who's attempted to capitalize on Collins' controversial decisions.

Gideon has won the endorsements of major progressive groups and has repeatedly led Collins in recent polling.

Here are some things to know about the Maine Democrat:

She's pursued progressive priorities as Maine's speaker of the House

Born in Rhode Island, Gideon went to college in Washington, D.C., and eventually wound up in Freeport, Maine, in 2004. Within about seven years, she ascended from serving on the Freeport Town Council to leading the state's House of Representatives.

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Since taking the speakership in 2016, Gideon has advanced reportedly bipartisan legislation on poverty. On climate change, she's at odds with President Trump in her support for rejoining the Paris Climate Agreement. Gideon also pushed legislation that requires her state to reach 80% renewable energy by 2030 and 100% by 2050. And on abortion, she sponsored legislation allowing physician assistants and registered nurses to perform the procedure.

She supports single-payer health care and is open to repealing the Senate's filibuster to pass it

The Senate race in Maine is important to watch for a number of reasons in a highly polarized environment like Washington, D.C. A Gideon victory would narrow Republicans' chances of keeping a majority and potentially make compromise even more difficult than it already is with a perceived moderate like Collins. 

The Maine Democrat has endorsed single-payer health care, putting her at odds with leadership in her party. As the 2020 Democratic primaries showed, the party is deeply divided on health care and those divisions could become magnified if they acquire enough power to pass substantial reform.

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Gideon told the Bangor Daily News she would change the 60-vote hurdle to a simple majority if it stood in the way of Democratic priorities, such as health care.

“I think what Americans need and what Mainers need more than anything is government that functions, and I think that the filibuster prevents us from functioning and making progress on issues,” she said. Collins has defended the procedural move and rebuked Gideon’s stance through a spokesperson.

Her fundraising has come under ethical scrutiny

Gideon received reimbursements from her Gideon Leadership PAC after she donated thousands of dollars to both the state's Democratic Party and a congressional campaign -- indicating she violated campaign finance law by serving as a proxy for a PAC's contribution. According to the Foundation for Accountability and Civic Trust (FACT), the campaign and the party both created additional violations when they failed "refund or disgorge" the contributions within 30 days of learning about them.

A spokesperson for Gideon's campaign previously said the committee was given incorrect guidance.

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"The contributions were within the legal limit and fully disclosed, but the committee was given incorrect guidance on how to process them. The error was immediately addressed," the spokesperson said.

The Maine Ethics Commission unanimously voted last October to impose a $500 fine on her now-shuttered political action committee.

Fox News' Morgan Phillips contributed to this report.