Sen. Rob Portman, a top Republican who was vetted to be Mitt Romney's running mate, announced that he now supports gay marriage -- and had changed his position after his son came out.

Portman announced his change of heart in the pages of the Columbus Dispatch [4]. The Ohio senator said he decided his Christian beliefs and conservatism are in line with his support of gay marriage.

"Ultimately, it came down to the Bible’s overarching themes of love and compassion and my belief that we are all children of God," he wrote, later adding: "We conservatives believe in personal liberty and minimal government interference in people’s lives. We also consider the family unit to be the fundamental building block of society. We should encourage people to make long-term commitments to each other and build families, so as to foster strong, stable communities and promote personal responsibility."

The issue of same-sex unions has energized Hollywood, especially since 2008's Proposition 8 banned them in California. An appelate court found the ban unconstitutional, and the Supreme Court is expected to rule on the issue.

Portman's new position is similar to that of former Vice President Dick Cheney, who has an openly gay daughter. They are add odds with their party's stance that marriage should be allowed only between a man and a woman.

Portman said in an interview with CNN [5] that he told Romney and his advisors that his son is gay. But he said they told him it wasn't a deal breaker.

Nonetheless, he was passed over for another advocate of fiscal conservatism, Rep. Paul Ryan.

Portman said his position has shifted since his son, Will, came out two years ago while a colllege freshman.

"We were surprised to learn he is gay but knew he was still the same person he’d always been," Portman said. "The only difference was that now we had a more complete picture of the son we love."

Portman also said he does not believe churches should be forced to perform same-sex marriages. He also signaled that while he supports gay marriage, he doesn't intend to push for federal legislation on the issue.

Possibly referring to the Proposition 8 case, Portman also said an "expansive court ruling" would "run the risk of deepening divisions rather than resolving them."

"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," he wrote.

Watch the CNN interview: