Sen. Lisa Murkowski (R-Alaska) on Wednesday became the third Republican in the upper chamber to back same-sex marriage.
"I am a life-long Republican because I believe in promoting freedom and limiting the reach of government. When government does act, I believe it should encourage family values," Murkowski said in a statement posted on her website.
"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."
Sens. Mark Kirk (Ill.) and Rob Portman (Ohio) are the two other Senate Republicans who have backed same-sex marriage. A dozen Democrats have made the same switch amid a broader shift in popular opinion.
Murkowski had hinted that she was moving toward supporting same-sex marriage, saying in March to a local Alaska newspaper that her views were "evolving."
"The term 'evolving view' has been perhaps overused, but I think it is an appropriate term for me to use," Murkowsi said at a local event according to the Chugiak-Eagle River Star.
Murkowski's announcement comes as the Supreme Court prepares to rule on the constitutionality of the Defense of Marriage Act and California's Proposition 8, which bans same-sex marriage in the state.
The pro-same sex marriage Human Rights Campaign praised Murkowski's move.
"Sen. Murkowski's courageous and principled announcement today sends a clear message that marriage equality must come to all 50 states in this country," HRC President Chad Griffin said in a statement.
"As the Supreme Court prepares to rule in two landmark marriage cases this month, a growing bipartisan coalition is standing up for the right of all couples to marry — and there is no turning back that tide."