Sen. Rob Portman, R-Ohio, told reporters from the Columbus Dispatch and other Ohio newspapers that his change of heart on the hot-button issue came two years after his son, Will, told him and his wife that he is gay.
"It allowed me to think of this issue from a new perspective, and that's of a Dad who loves his son a lot and wants him to have the same opportunities that his brother and sister would have—to have a relationship like Jane and I have had for over 26 years," Portman said.
In an interview with CNN, Portman said his son, then a freshman at Yale University, told him "that he was gay, and that it was not a choice, and that it's just part of who he is, and that he'd been that way for as long as he could remember."
The dramatic announcement comes just a week before the U.S. Supreme Court is to hear oral arguments on the Defense of Marriage Act, which bars federal recognition of same-sex marriage, a measure Portman co-sponsored as a member of the House in 1996.
Portman was on the short list to be Mitt Romney's running mate last year. He said he informed Romney during the vetting process that he had a gay son, but that the issue was not a deal breaker for Romney.
He also told CNN that he sought guidance from former Vice President Dick Cheney, whose daughter Mary is gay.
Portman said Cheney's advice to him was simple: "Follow your heart."