He said he had talked to his pastor and others, including Senate Minority Leader Mitch McConnell, R-Ky., who opposes gay marriage, and to former Vice President Dick Cheney, who supports it.
Cheney, whose younger daughter is a lesbian, became arguably the best-known Republican to embrace gay marriage with his announcement in June 2009.
Portman said his previous views on marriage were rooted in his Methodist faith.
However, he wrote, “Ultimately, for me, it came down to the Bible’s overarching themes of love and compassion and my belief that we are all children of God.”
Despite his party’s struggles with Americans’ increasing acceptance of gay rights, many GOP leaders met Portman’s news with silence or a shrug.
A spokesman for House Speaker John Boehner, who shares Portman’s Cincinnati background, said the senator “is a great friend and ally, and the speaker respects his position, but the speaker continues to believe that marriage is between a man and a woman.”
In January, Boehner chastised the Obama administration for dropping its legal defense of the Defense of Marriage Act, which the Supreme Court is to consider this month. Boehner authorized the continued use of public funds to defend the law in courts.
Boehner’s latest comments reflect the change among many mainstream Republicans, who now deal with gay marriage in largely unemotional, legalistic terms rather than emotional terms about sin and God’s will.
Congress now has several openly gay members, including a senator, Wisconsin Democrat Tammy Baldwin.
At the U.S. House, which was in session Friday, several conservatives had little or nothing to say about Portman’s announcement.
Gay marriage “is not the most front-burner issue,” said Rep. James Lankford, R-Okla. “We still have the same legal issues we’ve always had with the Defense of Marriage Act,” which he supports, he said.