With Americans tilting toward support of gay marriage and two GOP senators now in favor, Republicans find themselves in a tightening political vice on the issue ahead of mid-term elections and the 2016 presidential race.

Last year was a watershed of sorts for the movement, with gay marriage laws passing in three states, Democratic President Barack Obama offering his public endorsement of marriage equality, and Wisconsin electing Tammy Baldwin as the first openly gay US senator.

But same-sex marriage is suddenly, unavoidably in the political spotlight once again, with the US Supreme Court mulling whether to repeal the Defense of Marriage Act which restricts federal benefits to marriages between a man and a woman.

And with the number of US senators backing gay marriage roaring past the halfway mark this past week -- 53 of 100 members are now in favor -- activists say Republicans risk getting left in the movement's wake, which could find them struggling to attract new voters.

"The reality is, there is now irrefutable momentum in the country" in favor of marriage equality, Evan Wolfson, a founder of the gay marriage movement in the United States and president of the non-partisan group Freedom to Marry, told AFP.

With each passing year the support for gay marriage grows greater and broader, with a solid 58 percent now in favor, according to the latest Washington Post/ABC News survey.

Wolfson said "true opposition to gay marriage is dwindling and isolated to a few demographic groups" -- namely Americans over 65, non-college-educated whites, and white evangelical Christians.

Young Republicans are siding with Democrats on the issue. Conservative freshman Senator Jeff Flake of Arizona even conceded last week it was "inevitable" that a future Republican presidential nominee would be in favor of marriage equality.

That puts Republicans in a pickle, especially after party leaders conducted a brutal self-criticism in the wake of their 2012 election debacle and announced they must do more to attract minorities like Hispanics.

Conservatives promote family and traditional values in their political platform. A Republican White House hopeful who openly espouses same-sex marriage could alienate the party's base, while opposing it could trigger charges he or she is behind the times.

Some Republicans are not shying away from pressing their case, namely that same-sex unions dilute the importance of marriage as the traditional avenue for raising children with both a mother and father.

But their alarmingly off-script statements are "sending a shudder through the coroners who just finished the Republican autopsy," noted Wolfson.

Among them are remarks by Tea Party-favorite Louie Gohmert, a congressman from Texas who mentioned homosexuality in the same breath as polygamy and bestiality.

"When you say it's not a man and a woman anymore, then why not have three men and one woman or four women and one man?" Gohmert told supporters on a recent conference call.

"Or why not, you know, somebody has a love for an animal or -- there is no clear place to draw a line once you eliminate the traditional marriage."

Wolfson dismissed Gohmert as an extremist voice who operates on the fringes and not in the weighty conservative center.

"But if that's the kind of voice the Republican Party puts forward, they're going to continue to sink like a stone," he said.

Republican leaders like House Speaker John Boehner and his deputy Eric Cantor continue to speak about big-tent inclusion, even while expressing their own personal opposition to gay marriage.

Potential Republican candidates for the White House in 2016 -- Senator Marco Rubio and New Jersey Governor Chris Christie, to name a few -- are opposed as well, while another possible candidate, former Florida governor Jeb Bush, has said it should be left to the states, nine of which now allow gay marriage.

"I could not imagine somebody who supports same-sex marriage winning the Republican nomination for president," said Joseph Backholm, executive director of the Family Policy Institute of Washington which supports traditional unions.

Likewise, a Democrat would need to back gay marriage -- and support a woman's right to abortion -- in order to be accepted by the party machinery, he noted.

"We're in an emotional debate," Backholm said, and today's climate has allowed Republicans like Senators Rob Portman and Mark Kirk to switch their positions to support gay marriage.

Most rights advocates insist the arc of history points toward broad acceptance of gay marriage, but Backholm believes the "emotional leverage" will fade, and the country will reconsider its current trend.

"There is no chance that this issue becomes uncontroversial any time soon," he said.