Washington (CNN) -- You probably recognize Sen. Rob Portman from his tireless campaigning for Mitt Romney in 2012. He was even on the short list to be Romney's running mate.

He's been a leading Republican voice on economic issues for four decades.

Now, the prominent Ohio conservative will be known for something else: reversing his hardline position against gay marriage.

He invited CNN to his Senate office to reveal the news.

"I'm announcing today a change of heart on an issue that a lot of people feel strongly about that has to do with gay couples' opportunity to marry," Portman told CNN.

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It has to do with another revelation, one deeply personal. His 21-year-old son, Will, is gay.

"I've come to the conclusion that for me, personally, I think this is something that we should allow people to do, to get married, and to have the joy and stability of marriage that I've had for over 26 years. That I want all of my children to have, including our son, who is gay," said Portman.

Will Portman told his father and mother he is gay two years ago, when he was a freshman at Yale University.

"My son came to Jane, my wife, and I, told us that he was gay, and that it was not a choice, and that it's just part of who he is, and that's who he'd been that way for as long as he could remember," said Portman.

What was the Republican senator's reaction?

"Love. Support," responded Portman.

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And complete surprise. He told CNN that he never suspected that his son was gay.

Portman says his son, now a junior in college, helped him work through his decision to announce his change in position on gay marriage and blessed the idea of publicly announcing Will Portman's sexuality.

"I think he's happy and, you know, proud that we've come to this point, but he let it be my decision just as you know, it's going to be his decision as to the role he plays going forward in this whole issue," said Portman.

Until now, this was a secret to most people in politics, but not everyone.

Last year, when Romney was vetting Portman to be his running mate, the Ohio Republican informed both Romney and his top campaign advisers that he has a gay son.

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"I told Mitt Romney everything," said Portman with a laugh. "That process is, intrusive would be one way to put it. But, no, yeah, I told him everything."

Portman, who was ultimately passed over as the GOP vice-presidential candidate in favor of Rep. Paul Ryan, said the fact that his son is gay was not the deal breaker for Romney. How does he know?

"Well, because they told me," said Portman.

Portman told CNN he sought counsel from a Republican who did serve as vice president: Dick Cheney, the highest-ranking Republican with an openly gay child, his daughter Mary.

"I spoke to him personally; I actually met with him," said Portman.

He said Cheney's advice was simple: "'Follow your heart.'"

"He was a good person to talk to because he also was surprised by the news, in that case, you know, his wonderful daughter, who he loves very much. And it forced him to re-think the issue too, and over time, he changed his view on it," said Portman.

"I followed his advice. You know, I followed my heart," he said.

Though he is a staunch conservative, Portman was never outspoken against gay marriage. But he consistently voted against it.

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While in Congress, he supported a constitutional amendment to ban gay marriage, voted for the Defense of Marriage Act and voted for a bill prohibiting gay couples in Washington from adopting.

In 2011, 100 University of Michigan law school graduates walked out of Portman's commencement address to protest his position on gay rights after circulating a petition trying to get him removed as the event's speaker.

"The decision to host a graduation speaker who is openly hostile to LGBT rights is deeply unfair to the LGBT students who will be in the audience this year celebrating their graduation," read the petition.

Portman admitted that when the protest occurred, he already knew his own son was gay.

"It was a little odd. Look, and to be honest with you, it didn't affect that decision-making much, because it was, I didn't think it was appropriate," Portman said. "But look, they had their freedom of speech rights to be able to do what they wanted to do."

"But you know, what happened to me is really personal. I mean, I hadn't thought a lot about this issue. Again, my focus has been on other issues over my public policy career," said Portman.

Asked about why he is announcing this change of heart, since his son revealed it to him two years ago, Portman cited two reasons. He just recently became comfortable with his decision to shift his position on gay marriage, and also he said he knew the Supreme Court is considering a pair of gay marriage cases, and reporters would likely ask him for his position.

"I thought it was the right time to let folks know where I stand so there's no confusion, so I would be clear about it," said Portman.

What would Portman say to gay constituents who may be glad he's changing his position on gay marriage, but also wondering why it took having a gay son to come around to supporting their rights?

"Well, I would say that, you know, I've had a change of heart based on a personal experience. That's certainly true," he responded with a shoulder shrug.

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But he also repeated a reality. His policy focus has been almost exclusively on economic issues.

"Now it's different, you know. I hadn't expected to be in this position. But I do think, you know, having spent a lot of time thinking about it and working through this issue personally that, you know, this is where I am, for reasons that are consistent with my political philosophy, including family values, including being a conservative who believes the family is a building block of society, so I'm comfortable there now."

To be sure, Portman was anything but comfortable discussing something as private and personal as his son's sexuality, even noting how foreign it felt for someone with his Midwestern upbringing and sensibilities to talk publicly about such issues.

At times, the press-savvy politician even seemed to tremble a bit. But it was also clear he was willing to endure the discussion in order to publicly tell his son that he is proud.

"He wanted to tell us that there's something about him that we didn't know," Portman said, reflecting on the day two years ago his son Will first told his parents he was gay.

Portman quickly added that it "of course hasn't changed our view of him at all."