(CNN) -- Republican Sen. Lisa Murkowski of Alaska announced her support Wednesday for same-sex marriage, becoming the third sitting GOP senator to back the right for gay and lesbian couples to marry.
She made her endorsement during an interview with a local television station in Alaska and in an opinion piece on her website, just days before the Supreme Court is expected to rule on two high-profile cases involving same-sex marriage.
"I support the right of all Americans to marry the person they love and choose because I believe doing so promotes both values: it keeps politicians out of the most private and personal aspects of peoples' lives -- while also encouraging more families to form and more adults to make a lifetime commitment to one another," she wrote.
Human Rights Campaign President Chad Griffin applauded the senator, describing Murkowski as "courageous and principled."
"We hope other fair-minded conservatives like Senator Murkowski stand up and join her. Alaska may be nicknamed 'the Last Frontier,' but we've got to make sure that LGBT Alaskans don't have to wait to find justice," he said in a statement.
The Supreme Court is expected to rule this month on the constitutionality of the Defense of Marriage Act and California's same-sex marriage ban known as Prop 8.
Earlier this year, Sen. Rob Portman of Ohio endorsed same-sex marriage, citing his son, who his gay, as a major influence on his decision. Sen. Mark Kirk of Illinois followed suit a month later.
Murkowski, who's not up for re-election until 2016, told local media in March she was "evolving" on the issue and was considering her stance "very closely." Her comments came two weeks after Portman made his announcement.
Meanwhile, a small group of Democrats in the Senate also voiced support for the issue for the first time earlier this year when the Supreme Court heard oral arguments on the two cases.
A majority of Americans support allowing same-sex couples to marry, with 57% in favor and 40% opposed, according to a recent ABC News/Washington Post poll. About a third of Republicans and 71% of Democrats support the issue.
"I think you are seeing a change in attitude, a change in tolerance," she told Alaska Public Media in March. "And an acceptance that what marriage should truly be about is a lasting, loving committed relationship with respect to the individual."
In 1998, Alaska became one of the first of now-29 states to pass an amendment to its constitution defining marriage as between a man and a woman.
Murkowski backed similar proposals on the federal level in 2004 and 2006.
"It may be that Alaska will come to revisit its position on gay marriage, and as a policy maker I am certainly reviewing that very closely," she said.
CNN's Ashley Killough, Martina Stewart and Paul Steinhauser contributed to this report.