Former Maine gubernatorial candidate Betsy Sweet announced Thursday she is challenging Sen. Susan Collins Susan Margaret CollinsThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Billionaire who donated to Trump in 2016 donates to Biden Credit union group to spend million on Senate, House races MORE (R-Maine) for her Senate seat.

Sweet, who has directed the Maine Women’s Lobby and the Maine Commission for Women, expressed support for abortion rights, health care for all and fighting climate change in her announcement speech.

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“Our division isn’t between our neighbors on the left and right,” she said in the speech, according to prepared remarks. “The division is between us and them – between working people and the rich and powerful elite who are lining the pockets of politicians and putting the needs of Maine families at the bottom of the list.”

She was endorsed by the progressive group Democracy for America, whose CEO Yvette Simpson called Sweet “the kind of bold leader who can harness the grassroots energy that’s been growing in Maine for years," in a statement.

Collins, who is in her fourth term, is one of two GOP senators running for reelection in a state Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE carried in 2016. A moderate, she is considered by many to be hard to beat.

The National Republican Senatorial Committee in a statement slammed Sweet as a "radical left-wing activist," touting Collins for her bipartisan record in the Senate.

"Susan Collins has the most bipartisan record in the U.S. Senate because she works with members of both parties to deliver results for Maine families," said NRSC spokesperson Nathan Brand. "Meanwhile, lobbyist Betsy Sweet is a radical left-wing activist committed to making the loony policy dreams of Nancy Pelosi Nancy PelosiPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Trump signs largely symbolic pre-existing conditions order amid lawsuit MORE a reality."

But she came under fire for her vote to confirm Supreme Court Justice Brett Kavanaugh last year amid fears a conservative majority on the court would threaten Roe v. Wade. She's become a target of progressive groups for that judicial vote and others, though last month she spoke out against the restrictive abortion laws being passed in states across the U.S.

The Cook Political Report rates her race as leaning Republican, putting her in a more vulnerable category than most Republican senators up for reelection, though in safer territory than Sens. Martha McSally Martha Elizabeth McSallyThe Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' New ABC/WaPost poll finds Trump edging Biden in Arizona, Florida MORE (R-Ariz.) and Cory Gardner Cory Scott GardnerBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Cook Political Report shifts Colorado Senate race toward Democrat MORE (R-Colo.).

Other Democrats have entered or are also mulling a challenge to Collins, with state House Speaker Sara Gideon (D) seen as the likeliest Collins challenger.

Updated 6:48 p.m.