Senator Rob Portman of Ohio has revealed that he's backing same-sex marriage—a reversal from his previous political history which includes votes against same-sex marriage and gay adoption. Portman says the reason for his change of heart is his own 21-year-old gay son, who came out to him two years ago, and he becomes the first Republican Senator on that side of the controversial issue as it heads for landmark arguments in the Supreme Court.

Portman, who was touted as a a potential running mate for Mitt Romney last year, says his son Will revealed his sexual orientation to his parents in February 2011. That was about three months before Portman would see around 100 members the University of Michigan law school's graduating class walk out of his commencement speech in protest of his anti-gay record. That record includes a 2004 vote for a constitutional amendment banning gay marriage, and in 1999, support for a bill that would bar gay and lesbian couples from adopting.

Just this past election year, Portman was vetted by Romney's campaign team as a vice-presidential option. According to the Columbus Dispatch's Jack Torry and Jessica Wahrman, Portman told Romney about his gay son, who didn't consider it an issue—although we went on to see Romney and Paul Ryan run on a platform that included a promise to defend the Defense of Marriage Act and the idea of "a constitutional amendment defining marriage as a union between one man and one woman," as ABC News reported at the time. It's not known if Portman discussed the issue of same-sex marriage with Romney or, if he had been picked, if he would have backed those initiatives despite having a gay son.

In an op-ed for the Dispatch in Friday's paper, Portman says the admission by Will, a junior at Yale,  lead to the reversal. Portman writes that his son helped him see that gay people don't choose to be gay:

Two years ago, my son Will, then a college freshman, told my wife, Jane, and me that he is gay. He said he’d known for some time, and that his sexual orientation wasn’t something he chose; it was simply a part of who he is. Jane and I were proud of him for his honesty and courage. We were surprised to learn he is gay but knew he was still the same person he’d always been. The only difference was that now we had a more complete picture of the son we love.

Portman adds that he's voicing his support for gay marriage now because the Supreme Court will be hearing arguments soon on California's Prop. 8, a state ban on same-sex marriage, as well as the constitutionality of the Defense of Marriage Act. According to The Advocate's Lucas Grindley"He is now the only Republican currently in the Senate to support marriage equality, but he's not the only one in Congress. Florida Rep. Ileana Ros-Lehtinen, who has a transgender son, in 2011 became the first GOP congresswoman to support repeal of the Defense of Marriage Act."