Thom Watson, right, helps his husband Jeff Tabaco with a flower before participating in a sit-in protest for same-sex couples that were denied marriage licenses from the San Francisco county clerk in this Feb. 14, 2013 photo in San Francisco. Credit: Justin Sullivan/Getty Images

Sen. Rob Portman's public and carefully choreographed endorsement of gay marriage is something of an anomaly, at least among Republicans. He's the third Republican now sitting in federal office to endorse same-sex marriage.

More than 100 high-profile Republicans recently urged the Supreme Court to strike down California's Prop. 8 and allow gay marriage in that state. But only two of them - Rep. Richard Hanna, R-N.Y., and Rep. Ileana Ros-Lehtinen, R-Fla. - are in federal elective office. If Republicans are going to keep pace with public opinion, he won't be the last.

After years of states' passing constitutional bans prohibiting gay marriage, public opinion has shifted and if a conservative lawmaker from a state whose same-sex marriage ban helped hand George W. Bush a second term can support same sex-marriage, it means the floodgates are opening.

The Supreme Court is also set to consider same-sex marriage. In two days of oral arguments later this month, the court will consider challenges both to Prop. 8, which outlawed same sex-marriage in California, and to the 1996 Defense of Marriage Act, which Portman co-sponsored as a congressman. He now thinks parts of the law should be repealed.

Foster Friess, a Republican megadonor, accepted an award at the Conservative Political Action Conference today standing next to Rick Santorum, a favorite of social conservatives for whose presidential candidacy he backed with millions of dollars last year. Friess stopped short of endorsing gay marriage, but he recently said the party has to move away from "gay bashers" and allow same-sex couples, at the very least, the same rights in the tax code as the rest of Americans.

RELATED: GOP's Rob Portman Endorses Gay Marriage

There have been other high-profile Republican supporters of same-sex marriage. California Gov. Arnold Schwarzenegger fought against Prop. 8, which banned same-sex marriage in his state in 2008.

A Relevant Issue?

Vice President Dick Cheney, who, like Portman, has a homosexual child, said he thought people should be able to enter into any kind of relationship of their choosing during the 2004 presidential election. But he clarified that he wasn't speaking for President George W. Bush, who opposed same-sex marriage.

So opposition to same-sex marriage was the official Bush White House position. Indeed, a state ballot initiative that year in Portman's home state of Ohio is credited with helping get out conservative voters and swing the state toward Bush. The state constitution in Ohio bans gay marriage as a result.

"Only a union between one man and one woman may be a marriage valid in or recognized by this state and its political subdivisions," according to the 2004 amendment to Ohio's constitution. "This state and its political subdivisions shall not create or recognize a legal status for relationships of unmarried individuals that intends to approximate the design, qualities, significance or effect of marriage."

ABC News' Jonathan Karl asked Cheney in July whether gay marriage would be a relevant issue in the future.

"I don't know that it's relevant now. You know? There are a lot of big issues," Cheney said.

"Do you wish you had pushed a little harder on that issue?" Karl asked Cheney.

"Why? … If I was out there 12 years ago in the first campaign - in 2000 - in the debate with Joe Lieberman in front of millions of Americans on live television and I laid out my position then and it hasn't changed, no. I've addressed it and moved on.

"The fact of the matter is that it is regulated by the states," Cheney said during that debate. "I think different states are likely to come to different conclusions, and that's appropriate. I don't think there should necessarily be a federal policy in this area. I try to be open minded about it as much as I can and tolerant of those relationships."

Thirty-one U.S. states have some kind of provision in their state constitution that bans same-sex unions. Twenty-eight of them have passed such measures since Cheney said he would try to be tolerant of same-sex relationships. Just nine states and the District of Columbia allow gay marriage.

A National Shift

But now the entire country seems to be in the midst of a shift on the issue. An ABC News-Washington Post poll in May of 2012 found 53 percent of Americans support gay marriage. That was up from 36 percent in 2006.

It was just one year ago that President Obama endorsed gay marriage. At the time he said the issue should be up to the states to decide. The president still says that, but he has added the caveat that he can't imagine a compelling justification for outlawing same-sex marriage.

But that shift is taking place slower among Republicans, who count social conservatives as part of their base.

Even still, you can sense a shift. Mitt Romney, Republicans' 2012 nominee for president, supported a constitutional amendment banning same-sex marriage. At least one potential candidate likely to get a good look in 2012, Sen. Marco Rubio, has said recently he thinks such an amendment would infringe on the rights of states.

"I've always been uncomfortable with a federal constitutional amendment on anything, particularly on that, because it steps on the rights of states to define marriage," he said during an interview with Buzzfeed at a Capitol Hill bar. "I think that's a two-way street, though. I mean, if states define marriage as between one man and one woman, you're going to say to a lot of states that you respect the decisions they make. That includes a state like Florida, which passed a constitutional amendment saying marriage is between one man and one woman."

Rubio urged "mutual respect" for those points of view at the CPAC conference for conservatives at the Gaylord National resort just outside Washington Thursday.

"Just because I believe that states should have the rights to define marriage in a traditional way does not make me a bigot," he said.

Sen. Rand Paul, R-Ky., who has given the GOP a libertarian shot in the arm of late, wants to get government out of the marriage business entirely.

"I'm an old-fashioned traditionalist. I believe in the historic and religious definition of marriage," he recently told National Review Online.

"That being said, I'm not for eliminating contracts between adults. I think there are ways to make the tax code more neutral, so it doesn't mention marriage. Then we don't have to redefine what marriage is; we just don't have marriage in the tax code."

Certainly there will be social conservatives still loudly opposing same-sex marriage in the party. But it is worth noting that even as their personal opinions differ, Rubio's policy position is not unlike what President Obama's was when he personally endorsed gay marriage in May of 2012. Obama said then that there was a healthy debate on the issue going on in the states.

Then, just after North Carolina had banned same-sex marriage, Obama said states should have the final word and he explained that he hadn't supported gay marriage earlier because he didn't want to "nationalize" the issue.

"I continue to believe that this is an issue that is going to be worked out at the local level, because historically, this has not been a federal issue, what's recognized as a marriage," Obama said in May of 2012. RELATED: President Obama Announces Support for Gay Marriage But even as the country and Republicans like Portman are evolving on the issue, President Obama seems to have further evolved. Obama told George Stephanopoulos this week that he can't envision a justifiable reason for a state to ban same-sex marriage. And his administration has joined the legal fight to overturn Prop. 8 in California. RELATED: President Obama Can't Imagine Justifiable Reason to Ban Gay Marriage