US Republican Senator Rob Portman favours gay unions
Influential US Republican Senator Rob Portman has renounced his opposition to gay marriage.
The Ohio senator said he began to change his mind in 2011 after his son, Will, revealed he was gay.
Sen Portman said that his former views on marriage had stemmed from his Methodist faith.
In 1996, as a member of the House of Representatives, he voted for the Defense of Marriage Act, which defines marriage as between a man and a woman.
He also voted in 1999 against allowing gay couples in Washington DC to adopt children.
The gay marriage issue has increasingly divided the Republican party, with some members arguing that opposition to same-sex unions contributed to the party's losses nationally in last November's elections.
Sen Portman, who was considered a potential running mate for Republican White House candidate Mitt Romney, announced his change of heart in interviews with several Ohio newspapers.
"I have come to believe that if two people are prepared to make a lifetime commitment to love and care for each other in good times and in bad, the government shouldn't deny them the opportunity to get married," he wrote in an opinion piece for the Columbus Dispatch.
"That isn't how I've always felt," he wrote. "As a congressman, and more recently as a senator, I opposed marriage for same-sex couples."
His son posted a message on Twitter, saying: "Especially proud of my dad today".
Sen Portman said to Ohio newspapers that his son Will, now 21 and a student at Yale University, told him being gay was "not a choice", and that he had been gay "since he could remember".
The Ohio senator said he and his wife, Jane, had been surprised but also supportive.
The father of three said he had confided in the pastor of his church in Cincinnati about the issue.
He also spoke to Senate Republican leader Mitch McConnell, who opposes gay marriage, and to former Vice-President Dick Cheney, who supports it. Mr Cheney's daughter is a lesbian.
Sen Portman told the newspapers that Mr Romney had been informed about Will's sexuality last year.
Mr Romney spoke out last May against gay marriage, a week after President Barack Obama took the political gamble of announcing his support for such unions.
Sen Portman said he had struggled with reconciling his Christian faith and his desire to see his son have the same opportunities for relationships as his siblings.
"Ultimately, it came down to the Bible's overarching themes of love and compassion and my belief that we are all children of God," the senator wrote.
Sen Portman said he would like Congress to repeal the provision of the Defense of Marriage Act (Doma), which bans federal recognition of gay marriage.
But he argued that states should not be forced to recognise such unions.
Sen Portman said in his opinion piece: "Judicial intervention from Washington would circumvent that process as it's moving in the direction of recognising marriage for same-sex couples.
"An expansive court ruling would run the risk of deepening divisions rather than resolving them."
Nine US states and the District of Columbia fully recognise gay marriage, including several states which legalised the unions through a popular vote in 2012.
The announcement comes just over a week before the US Supreme court will hear arguments in two cases related to gay marriage. One challenges Doma.