Earlier this month, Senator Rob Portman became the first Republican in the chamber to endorse same-sex marriage.
“The joy and the stability of marriage that I have had for 26 years - I want all three of my kids to have it, including our son, who is gay," he said.
The announcement, on CNN, did nothing to change the opinions of fellow-Republican senators like Orrin Hatch.
“We are friends [Portman and I]. But where we differ is I do not believe we should change the traditional definition of marriage," he said.
The cases before the Supreme Court include a challenge to the Defense of Marriage Act, which bars federal recognition of same-sex unions. The law, known as DOMA, received strong bipartisan support when it was enacted in 1996, including from then-Senator Robert Byrd, a Democrat.
“To insist that male-male or female-female relationships must have the same status as the marriage relationship is more than unwise. It is patently absurd," he said.
But others who voted for DOMA have had a change of heart. Democratic Senator Tom Harkin said, “It is not the only vote I regret, but it is one of them. It was not a good vote . I have changed my whole view on that completely.”
Public-opinion polls show a growing majority backing same-sex marriage rights. A decade ago, barely one-in-three Americans did so.
Democratic Senator Richard Durbin says, until recently, Republicans used the issue to hammer Democrats at the voting booth.
“We [Democrats] used to jokingly say that the campaign against all Democrats was on the issues of ‘God, gays, and guns’," he said.
More recently, the tide has turned. Democrats, including President Barack Obama, have won elections proclaiming support for same-sex marriage. Senator Durbin, who voted for DOMA in 1996, applauds the turn of events.
“Younger generations think that positions supporting marriage equality are more consistent with their values and vision of America. And Democrats have led in [reflecting] that, and maybe we will benefit [politically] from it. But at least many of us feel we are in the right position in terms of America’s values," he said.
To be clear, some Democratic lawmakers do not endorse same-sex marriage, and some Republicans are urging their party to rethink the issue.
University of Virginia political analyst Larry Sabato said, “Within the Republican Party, a majority still opposes same-sex marriage. It is a real dilemma for Republicans. It is a loser for them, and they know it. They cannot endorse it, because of the social conservatives. They cannot oppose it, because of their need for a broader constituency, to reach out to voters before they become a permanent minority party.”
Sabato says Democrats have reaped benefits from backing gay rights, and not just at the ballot box. “It has helped the Democrats, certainly in fundraising. The gay and lesbian population pours money into the Democratic Party," he said.
The Supreme Court could uphold anti-gay marriage laws or strike them down as unconstitutional. If struck down, the court could conceivably pave the way for same-sex marriage rights nationwide. Larry Sabato says such an outcome would reduce the political potency of the issue for Democrats and Republicans.