U.S. Sen. Susan Collins won her race for re-election against Democratic challenger Shenna Bellows on Tuesday. The Associated Press announced the results based on exit polls.

Collins employed a safe incumbent-defense strategy that emphasized the senator’s reputation as a bipartisan coalition builder and her familiarity with Mainers, perhaps best illustrated by her campaign slogan, “Susan Collins – our senator.” All the while, her campaign worked hard in social and traditional media to portray Bellows’ campaign as error-prone and the candidate as too liberal for Maine.

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Susan Collins gives her victory speech Tuesday night at the Westin Portland. The incumbent’s campaign emphasized her reputation as a moderate Republican. Gordon Chibroski/Staff Photographer

State Sen. Shenna Bellows, D-Manchester, sponsored a bill to prevent some internet service providers from selling users' private data. 2014 AP file

A moderate Republican who consistently ranks among the state’s most popular politicians, Collins faced an energetic and bold challenger in Bellows.

A former executive director of the American Civil Liberties Union of Maine, Bellows was not widely known outside of Maine political circles but entered the race as both an experienced policy debater and a veteran of hard-fought campaigns.

She helped lead Maine’s two ballot-box referendums to legalize same-sex marriage as well as a third campaign that restored Mainers’ ability to register to vote on Election Day. As the ACLU’s chief spokeswoman and lobbyist for eight years, Bellows was heavily involved in legislative battles over abortion, voting rights, civil liberties and free speech.

Bellows impressed outside observers with her ability to raise more than $2 million largely on her own, her 350-mile “Walk Across Maine” and her willingness to go after Collins. She used the five joint debates to challenge Collins about her votes on issues such as the minimum wage, the Affordable Care Act and gender discrimination – key issues on the Democrats’ national campaign agenda this year.

But Bellows’ campaign never picked up the critical support of large, national political groups often necessary to help challengers unseat a well-funded incumbent, much less one with substantial cross-party appeal like Collins.

Democrats’ two leading political action committees, the Democratic Senatorial Campaign Committee and the Senate Majority PAC, spent more than $100 million in more competitive races in hopes of retaining control of the chamber. Likewise, a number of high-profile organizations that typically lean left – such as the League of Conservation Voters and the Human Rights Campaign, the nation’s largest gay rights group – endorsed Collins because of her reputation as a fairly reliable moderate vote in an increasingly conservative Republican caucus.

Collins had raised more than $5.1 million as of Oct. 15 compared to $2.2 million for Bellows, according to financial reports filed with the Federal Election Commission.

At 39, Bellows is widely expected to run for political office again in Maine. When asked about it Tuesday night, though, she said, “Honestly, I haven’t thought beyond tonight.”

“We’re building a positive movement for change, and we’re in this for the long haul,” she said. “We are so proud of how far we came and how close we stayed to our values. This was an extraordinary grass-roots effort.”