Maine Republican Susan Collins publicly announced her support for gay marriage on Wednesday, following the endorsement of the nation’s largest LGBT advocacy group, the Human Rights Campaign.

Collins decision to publicly favor gay marriage – and the endorsement – comes as she faces a re-election challenge from the left, against former American Civil Liberties Union director Shenna Bellows who has put gay marriage and rights at the center of her campaign.

“A number of states, including my home state of Maine, have now legalized same-sex marriage, and I agree with that decision,” Collins told Maine’s Bangor Daily News.

Bellows slammed Collins in a statement for not speaking out sooner.

“Two years after her constituents made their feelings known at the ballot box, she has refused to break her silence. I believe Mainers need, want and deserve more proactive representation on equal rights—on allowing LGBT students to learn without fear of bullying, on applying for jobs and going to work without fear of discrimination, and on much more. I’m running for Senate to provide that proactive representation and to expand Constitutional protections for our LGBT community,” she said.

Collins joins Sen. Rob Portman of Ohio, whose son is gay, Sen. Mark Kirk of Illinois, and Sen. Lisa Murkowski of Alaska as the only Republican U.S. senators to publicly back marriage equality. Collins has long maintained the position that individual states should decide whether to legalize same-sex marriage, just as Maine did in 2012, one of the first states to do so by popular vote. She also voted twice against proposed amendments to codify marriage as a union between a man and a woman into the Constitition.

“What she has consistently said is she doesn’t want to get involved in state-level referendum issues. She’s a U.S. senator, and she stays within the purview of her office,” a spokesman for the Maine incumbent said. “But when asked [Wednesday] about her personal stance on this issue, she’s said she supports it.”

“Nearly 44% of Americans live in a state where same-sex couples can be legally married, and I believe this number will only continue to grow,” Collins told the Bangor Daily News on Wednesday.