AUGUSTA — After receiving the endorsement of the nation’s largest LGBT advocacy group, U.S. Sen. Susan Collins on Wednesday indicated for the first time her personal support for same-sex marriage.

“A number of states, including my home state of Maine, have now legalized same-sex marriage, and I agree with that decision,” Collins said in response to a question from the BDN.

It’s long been a matter of speculation whether Collins would be the next Republican lawmaker in Washington to publicly support same-sex marriage. She is just the fourth U.S. Senate Republican to do so, joining Sens. Rob Portman of Ohio, Mark Kirk of Illinois and Lisa Murkowski of Alaska.

Collins has long maintained that the question of same-sex marriage is one for each state to decide on its own.

Voters in Maine have been asked twice whether to legalize same-sex marriage. In 2009, they said no. Three years later, they said yes.

A spokesman for Collins’ re-election campaign, Lance Dutson, confirmed that she was publicly announcing her support for gay marriage for the first time. He said the senator hasn’t spoken publicly on the issue previously because she believes it is the voters of each state — not U.S. senators — who should make the decision on marriage equality.

“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.”

Since Massachusetts became the first state in the country to allow same-sex couples to marry in 2004, 18 other states have joined suit. Maine was the first state in the nation to approve same-sex marriage by popular vote.

“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,” Collins said Wednesday.

While Collins had never made her personal stance on same-sex marriage known, she voted twice — in 2004 and 2006 — against proposed amendments to the U.S. Constitution that would have codified in the highest law that marriage only existed between one man and woman.

The marriage question aside, Collins has widely been considered an ally of the lesbian, gay, bisexual and transgender communities. In 2010, she and U.S. Sen. Joe Lieberman, I-Connecticut, sponsored legislation to repeal the Pentagon’s ban on openly gay men and women serving in the military. The bill ultimately passed Congress and became law.

Collins also helped shepherd the Employment Non-Discrimination Act, or ENDA, through the Senate. That bill protects LGBT people from workplace discrimination.

On Wednesday, the Human Rights Campaign — the nation’s largest gay rights advocacy group — endorsed Collins’ re-election.

“Sen. Susan Collins has played a pivotal role in advancing support for LGBT equality,” said Chad Griffin, president of the Human Rights Campaign, in a news release. “HRC is proud to stand with Sen. Collins, and with allies on both sides of the aisle like her, because she firmly believes that every American should be evaluated based on their abilities, and not who they love.”

It’s the second time the group has endorsed Collins, but the endorsement is notable because her Democratic opponent — former American Civil Liberties Union of Maine director Shenna Bellows — is known for her role in Maine’s 2012 gay marriage referendum.

Bellows led Mainers United for Marriage, the 2012 coalition dedicated to winning over Mainers who three years earlier had voted to deny marriage to same-sex couples.

After hearing of Collins’ public support for marriage equality, Bellows on Wednesday said she thought the senator should have spoken up sooner.

“My opponent’s voice on marriage equality could have made a real difference in 2012, when this was up for a vote in Maine,” she said. “At that time, I was organizing Republicans for the Freedom to Marry, and we invited Susan Collins to endorse the campaign. She declined.”

Bellows has said that if elected, she will sponsor a bill to make same-sex marriage legal nationwide, as well as a national Human Rights Act, modeled on Maine’s Human Rights Act, to extend nondiscrimination protection to LGBT people in employment, housing, credit, public accommodation and education — with no religious exemption.

She also said she didn’t agree with Collins’ stance that senators should not take stands on referendums in their home states.

“I will never be silent when it comes to fundamental civil rights and constitutional freedoms,” she said.

Dutson dismissed criticism of Collins for not speaking out sooner.

In 2012, he said, “She was in Washington casting votes and working hard on all the issues important to the Human Rights Campaign.”

Collins’ support for same-sex marriage puts her out of step with the Maine Republican Party’s official stance, enshrined in its platform, that marriage exists only between one man and one woman.