Whether he wants to or not, Rob Portman has just become a leading player in the Republican Party’s socially moderate camp.

He says he’s ambivalent about embracing his new role, but the moment looks right for him on two fronts: There’s an unexpected opening in his schedule, and the party needs all the help it can get from its elder statesmen in catching up to the people it would presume to lead.

On Friday Portman became the first current GOP senator to take the liberal side in one of the defining social issues of our time, declaring he was converting to an embrace of same-sex marriage because one of his sons told him he is gay. In doing so, he created one of the biggest political and cultural Rorschach tests of the year for the in-search-of-its-new-self GOP — from the floor of CPAC to the bar at the Capitol Hill Club to the offices of the Log Cabin Republicans.

But for Ohio’s junior senator, the enlightened self-interest seems clear.

One of Portman’s big current assignments is helping to recruit and raise money for the sorts of establishment candidates who can win tossup Senate races in 2014 and ward off hard-right primary challenges along the way. His new stand should help with that.

At the same time, with the prospects dimming for either a big tax overhaul or a significant deficit-reduction package this year, Portman’s fiscal expertise and his seats on the Budget and Finance committees won’t generate the sort of attention he’d been expecting to help keep him viable as a national figure. But taking the lead in defining a new middle ground for his party on marriage equality stands to do precisely that.

Breaking the news in the middle of the Conservative Political Action Conference also allowed Portman’s message to trump the culture-warrior rhetoric infusing that meeting — including the declaration of another nationally ambitious GOP senator, Marco Rubio of Florida, that defending “marriage in the traditional way does not make me a bigot.”

Portman’s effect was to do his party more of a favor than anyone at that convention by offering a high-profile antidote to the current storyline about the Republicans: That they’re still highly resistant to embracing the need for a bigger tent four months after losing the popular vote for the fifth time in the past six presidential elections.

Beyond that, Portman synthesized as well as anyone the argument for why the right should support gay couples who want to get married — and did so just as a Supreme Court with a culturally conservative majority is preparing for oral arguments in its two landmark cases about the constitutionality of gay marriage.

“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,” Portman wrote in a Columbus Dispatch op-ed explaining his new thinking.  “One way to look at it is that gay couples’ desire to marry doesn’t amount to a threat but rather a tribute to marriage, and a potential source of renewed strength for the institution.”

One of the cases the court will hear next week will decide the future of the Defense of Marriage Act, and Portman will get plenty of attention by reiterating that he now views his vote for it in the House in 1996 as a mistake. But at the same time, he’ll encourage the court to rule narrowly against both that law and California’s Proposition 8 ban, arguing that the nation’s inevitable embrace of gay marriage will be more easily achieved through a series of statewide elections than under a judicial mandate.

His assessment is buttressed by the polls. The percentage who want to give lesbians and gays comprehensive marriage rights has doubled since DOMA was enacted and is now a clear majority. The principal divide over the issue is now generational, not partisan, and it’s younger voters in both parties who are overwhelmingly in favor.

Back in 2004, George W. Bush was re-elected after anti-gay marriage referenda were placed on the ballots in 11 swing states, all with the help of a Bush campaign apparatus. All were adopted, none more crucially than in Ohio, where the state tipped narrowly to the president because of a surge in conservative turnout in Portman’s own home base of Hamilton County.

As the senator knows as well as anyone, by the time he might run for president again, that scenario could never be repeated.