In an opinion piece yesterday in an Ohio newspaper, Mr Portman announced he now supports gay marriage. “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 op-ed piece in the Columbus Dispatch, titled “The Freedom to Marry”.
“That isn’t how I’ve always felt. As a congressman, and more recently as a senator, I opposed marriage for same-sex couples. Then something happened that led me to think through my position in a much deeper way.”
Mr Portman said his 21-year-old son, Will, told the senator and his wife that he was gay in February 2011.
Mr Portman’s announcement comes a week before the US supreme court is to hear arguments in two cases related to gay marriage.
One challenges the 1996 federal defence of marriage act, which defines marriage as a union between a man and a woman.
The court will also hear arguments that question a California law, known as Proposition 8, banning gay marriage.
Mr Portman told another Ohio newspaper, the Cleveland Plain Dealer, he now believes same-sex couples who marry in states where it is legal should be eligible for the same federal benefits granted to heterosexual couples.
The Republican Party is split on the gay marriage, with many arguing that socially conservative positions are contributing to the party’s election losses.