WASHINGTON - (AP) - Equivocal no longer, President Barack Obama declared his support for gay marriage on Wednesday in a historic announcement that instantly elevated a polarizing social issue to a more prominent role in the 2012 race for the White House. The announcement was the first by a sitting president, and Obama's Republican rival, Mitt Romney, swiftly disagreed with it. "I believe that marriage is between a man and a woman," Romney said from Oklahoma. Gay rights advocates cheered Obama's declaration, which they had long urged him to make. Joe Solomonese, president of the Human Rights Campaign, said it "extends that message of hope" to gays and lesbians. Obama announced his shift - he had said for years that his views on gay marriage were "evolving" - in an interview with ABC in which he cited a blend of the personal and the presidential. He said "it wouldn't dawn" on his daughters, Sasha and Malia, that some of their friends' parents would be treated differently than others, and added that he had thought of aides "who are in incredibly committed monogamous same-sex relationships who are raising kids together."