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.

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.

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.

"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.