Collins publicly announced her support for same-sex marriage after the Human Rights Campaign, America's largest LGBT advocacy group, endorsed her re-election over Democrat challenger Shenna Bellows.
"A number of states, including my home state of Maine, have now legalized same-sex marriage, and I agree with that decision," Collins explained to the Bangor Daily News.
"Nearly 44 percent of Americans live in a state where same-sex couples can be legally married, and I believe this number will only continue to grow."
Lance Dutson, a spokesman for Collins, said the senator has chosen to stay out of the debate for marriage equality because she did not want to overstep her legislative limits.
"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," Dutson said. "But when asked [Wednesday] about her personal stance on this issue, she's said she supports it."
Collins' opponent, former ACLU of Maine director Shenna Bellows, is critical of Collins' timing, citing her own experience with the senator and same-sex marriage.
"My opponent's voice on marriage equality could have made a real difference in 2012, when this was up for a vote in Maine," Bellows said.
"At that time, I was organizing Republicans for the Freedom to Marry, and we invited Susan Collins to endorse the campaign. She declined."