Jay Laprete / AP file Sen. Rob Portman at left, rides with his son, Will, in Ohio on Aug. 11, 2012.

Republican Senator Rob Portman has announced his support for same-sex marriage, saying he reversed his position on the divisive social issue after his son came out 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,” Portman wrote in an op-ed published Friday in the Columbus Dispatch.

“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 wrote in the op-ed.

The decision came after long consideration, the Ohio lawmaker told newspapers from his home state on Thursday. Portman’s 21-year-old son Will, who is a junior at Yale University, discussed his sexual orientation with Portman and his wife in 2011, the senator said.

His son said that his sexuality was “not a choice, it was who he is and that he had been that way since he could remember,” Portman told Cleveland.com during an interview in his Washington, D.C. office.

“It allowed me to think of this 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 told reporters during that interview.

Portman’s changed stance comes amid spreading support for same-sex marriage. Forty-eight percent of Americans supported same-sex marriage in 2012, up from 35 percent a decade ago, according to a Pew Research Center analysis from Dec. 2012.

Jeff Swinger / The Cincinnati Enquirer via AP, file U.S. Sen. Rob Portman is greeted by his daughter Sally, left, and son Will after finishing the Paddlefest, an annual Cincinnati event, on Saturday, June 23, 2012.

Arguments challenging a section of the Defense of Marriage Act, which Portman voted for in 1996, and the California constitutional provision that limits marriage to one man-one woman unions, are due to be heard before the Supreme Court later this month.

Portman called Speaker of the House John Boehner to tell Boehner of his decision to support same-sex marriage.

“Senator Portman 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,” said Michael Steel, a spokesman for Boehner.

In his commentary for the Columbus Dispatch, Portman said, “The process of citizens persuading fellow citizens is how consensus is built and enduring change is forged. That’s why I believe change should come about through the democratic process in the states. Judicial intervention from Washington would circumvent that process as it’s moving in the direction of recognizing marriage for same-sex couples. An expansive court ruling would run the risk of deepening divisions rather than resolving them.”

Ohio voters adopted an amendment to the state constitution in 2004 that said only a marriage between one man and one woman would be legally recognized in the state. The measure passed with 62 percent of the vote. According to Cleveland.com, Portman said that if Ohio voters were to reconsider that constitutional provision, “he might support it, depending on its wording ... .”

Portman was considered a potential vice presidential candidate to run with Mitt Romney in the last presidential election, and acted as a surrogate for the Romney campaign in the important swing state of Ohio.

Prominent Ohio conservative Senator Rob Portman, once considered for Mitt Romney's running mate, is speaking out about gay marriage in support of his son, who is gay, just as Romney gets set to give his first public speech since the election today. NBC's Peter Alexander reports.

Portman told Romney that his son was gay, the senator said in an interview with CNN that aired Friday.

“I told Mitt Romney everything,” Portman said of the vetting he underwent last year. “That process is, intrusive would be one way to put it. But, no, yeah, I told him everything.”

Mitt Romney adviser Beth Myers, who led the vice presidential vetting process, confirmed to NBC News Friday morning that that the campaign was aware Sen. Rob Portman's son was gay as they vetted him for the number 2 job – but that the situation played "no role whatsoever" in Romney's decision about who to pick as his running mate.

"It did not play a role," she said. Myers said that Portman told the campaign about the fact that his son was gay.

Myers said Portman called her on Thursday evening to tell her about his decision to switch his position and come out in support of gay marriage.

In the interview with Cleveland.com, Portman said that he believes the issue of same-sex marriage is “more generational than it is partisan.” He said that former Vice President Dick Cheney, whose daughter is a lesbian, told him to “do the right thing, follow your heart.”

Portman said he also considered his Christian faith, which led him to decide that “in a way, this strengthens the institution of marriage.”

“The overriding message of love and compassion that I take from the Bible, and certainly from the Golden Rule, and that fact that I believe we are all created by our maker, that has all influenced me in terms of my change on this issue,” Portman said, according to Cleveland.com.

“Especially proud of my dad today,” Will Portman tweeted on Friday with a link to the Columbus Dispatch op-ed.

The announcement came the same day as fellow Republican Senator Marco Rubio reiterated his opposition to same-sex marriage at a conservative gathering.

“Just because I believe that states should have the right to define marriage in a traditional way does not make me a bigot,” Rubio told the Conservative Political Action Conference on Thursday.

NBC News' Kasie Hunt and Frank Thorp contributed to this report.

Conservative Sen. Rob Portman, R-Ohio, reveals during an interview with CNN that he has changed course on his opinion of same-sex marriage, and he now supports it because his son has come out as openly gay.

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