As Republican politicians incrementally adjust their rhetoric on gay marriage, a pivotal political question simmers beneath the surface: Could a pro-gay marriage candidate capture the party’s presidential nomination in 2016?

At first blush, it seems hard to imagine.

Some of the activists who wield considerable influence over conservatives in early states are already warning it’s a bridge too far.

Craig Robinson, who runs The Iowa Republican blog told The Daily Beast that if New Jersey Gov. Chris Christie were to sign a gay marriage law, “it would almost disqualify him as a national candidate.”

In an interview with The RUN, the Iowa Family Leader’s Bob Vander Plaats said he doesn’t see a proponent of gay marriage surviving the rough-and-tumble primary process.

Yet, as they witness a sea change in public opinion before their eyes, GOP operatives realize it’s less a matter of if than when. And more precisely, how fast?

There will be a GOP nominee for president who supports gay marriage eventually. What’s less clear is if it could happen as soon as 2016.

Karl Rove, the uber-strategist who used opposition to marriage equality to browbeat Democrats a decade ago, now says he could envision the possibility.

And there’s a counter-intuitive case to be made that a 2016 Republican could actually benefit from coming out for marriage equality.

“The one thing that’s clear is every single demographic group has increased their support for gay marriage. Even conservatives and Republicans,” said Alex Lundry, a GOP pollster who served as Mitt Romney’s director of data and strategy.

Support for same-sex marriage among Republicans is up 18 percent over the past nine years (to 33 percent), according to the latest ABC/Washington Post poll. Among evangelicals, the increase is 24 percent.

The generational shift is even more profound in the survey. Among evangelical millennials (born between 1980 and 2000), 64 percent support gay marriage. Republican leaners (swing voters inclined to end up in the GOP column) under the age of 50 come in at 52 percent support.

By 2016, Lundry calculates that millennials will be the largest generation represented in the country’s voting age population.

“At the rate of change we have now, the trend lines are going nowhere but up. If you pull these trend lines out four more years, I think there can be a path,” said Lundry when asked if a Republican embracing gay marriage could clinch the nomination.

Of course, primaries aren’t won nationally — they’re clinched through the state-by-state slog, beginning in Iowa and New Hampshire.

That’s why the groundwork in a state known for its Christian conservative-traditional value primary pool is already well underway.

The pro-gay marriage group Iowa Republicans for Freedom has a goal of identifying 40,000 GOP caucus-goers in favor of marriage equality by 2016.

Given that the GOP caucuses usually draw a total of about 120,000 — nailing down a third of the vote on the side of gay marriage would drive home a salient political point.

“That’s huge. It could be defining,” said GOP political strategist Margaret Hoover, who helped rally Republican support for New York’s law recognizing same-sex marriage.

Hoover is absolutely convinced that one if not more of the top Republican candidates will switch their position by time 2016 rolls around.

“There won’t be a monolithic traditional marriage represented on the primary stage,” she predicted. “Even look at Marco Rubio’s rhetoric. He’s starting to water it down.”

And Iowa — which passed gay marriage in 2009 — presents the hardest slog for Republicans, and hardly has a track record of being determinative.

In New Hampshire, the atmosphere is even riper for a pro-gay marriage contender.

Last March, a majority of state House Republicans voted to kill a measure to repeal gay marriage in the Granite State.

For Tyler Deaton, the chairman for the Young Conservatives for Freedom to Marry, it was a moment that showed the pendulum had swung.

“That gives you an indication of where the Republican Party is at in New Hampshire,” said Deaton, a four-year resident of New Hampshire. “For many people this is a settled issue and it distracts from other issues that even the more conservative Republicans would like to talk about it.

Added Hoover, “”Marriage is a settled issue in New Hampshire because of Republicans.”

A year ago, GOP pollster Jan van Lohuizen found that among likely primary voters there, support was evenly divided on the law: 47 percent supported leaving it as is and 47 percent supported repeal.

“You’ve got to add points to that,” noted Deaton. “Of course, as time passes, these numbers only improve.”

Deaton also warned prospective candidates that when they come looking for ground forces to knock doors, man phones and place signs, it’ll be an important question for the youngest generation.

“Young Republican volunteers here want a candidate who can be trusted on Freedom to Marry,” said Deaton, who believes there will be a top tier Republican who comes out in support for gay marriage.

Finally, there’s the money.

There are a slate of deep-pocketed Republicans donors in the northeast corridor who hold marriage equality as one of their top pet issues.

Hedge fund managers like Paul Singer, Daniel Loeb and David Tepper have all poured their money into pro-gay marriage causes, including the effort to legalize gay marriage in New York.

It may not be a dealbreaker once Rubio or Christie come calling, but Hoover said these mega-donors have “let candidates know they care about this issue.”

Another GOP operative put it more bluntly: “The vast majority of people who will write $1 million-plus checks to a Super PAC are pro-gay marriage. In 2016, if you can’t fund a Free and Strong America PAC style Super PAC you won’t be able to win.”

So far, the only potential GOP candidate who has endorsed gay marriage is former Utah Gov. Jon Huntsman. Rubio, Sen. Rand Paul, and Gov. Jeb Bush are sticking to their guns for now, couching their position in a states rights stance.

It’s impossible to predict if one of them will go through a personal transformation a la Sen. Rob Portman.

But it’s likely that some of them will at least broach conversation with their closest political aides at some point about the political ramifications of being left behind cresting public opinion.

Lundry cautioned it would be foolish for any of them to change their mind without opening a dialogue with the more conservative wing of the party first.

“It’s not something I expect them to change on very quickly, very easily,” he said. “I have deep respect for their belief. But it’s not where the majority of the country is.”