Sen. Rob Portman (R-Ohio) says he has decided to support gay marriage, two years after his son came out to him as gay.

"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," the Republican, who was on the short list to become former Massachusetts governor Mitt Romney's running mate in the 2012 presidential campaign, wrote in an op-ed that appeared in the Columbus Dispatch on Friday.

"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."

Portman told local reporters that his 21-year-old son, Will, who is a student at Yale University, came out to him and his wife in February 2011.

"It allowed me to think about this issue from a new perspective, and that’s as a dad who loves his son a lot and wants him to have the same opportunities that his brother and sister have,” Portman said.

On Friday morning, his son responded:

Portman is the highest-ranking Republican, and conservative, currently in office to embrace gay marriage, a position that remains very unpopular with the conservative base. The senator also discussed his decision in an interview with CNN and with the Cincinnati Enquirer newspaper. A spokeswoman said Portman is traveling with his sons this weekend and does not plan to comment further.

While many Republicans and conservatives who don't hold office recently signed onto a brief urging the Supreme Court to legalize gay marriage, there were only two current Republican House members on the list -- Reps. Richard Hanna (N.Y.) and Ileana Ros-Lehtinen (Fla.).

Portman said his decision to change his position was part of a process of adjustment that he has undergone since learning his son is gay. He said he informed Romney's vetting team about his son's sexual orientation when Portman was being considered as a potential vice presidential pick.

Portman said he would like to see Section 3 of the federal Defense of Marriage Act, which defines marriage as a union of one man and one woman, repealed. He believes that the federal government should allow individual states to decide "through the democratic process" how to handle gay marriage.

Former president Bill Clinton, who signed the DOMA bill, announced last week that he has decided to support gay marriage and believes the federal bill should be overturned. President Obama changed his own position on gay marriage last year, saying he had been "going through an evolution" on the issue and had concluded same-sex couples should be able to marry.

Ed O'Keefe contributed to this report.