While the economy is certain to dominate the campaign over the next six months, gay marriage could have an impact at the margins in key states from Colorado to Ohio to Virginia by influencing voter turnout among important constituencies, among them minorities, young voters and evangelicals.
"It may cost you as many votes as it wins you," said Colorado Republican Greg Brophy, a state senator.
Advocates on both sides of the emotional issue agree Obama's pronouncement will stoke enthusiasm among core Democrats and Republicans, likely boosting turnout in the November election and fundraising ahead of it. The big unknown is where independent voters — and specifically those Obama struggles to win over, such as middle-class whites — land in the fewer than a dozen states expected to make a difference in the quest for the White House.
"Any little one thing could be the issue that turns Nevada one way or the other," said James Smack, a Republican National Committee member. He also could have been talking about the other states that are expected to be too close to call until the end, among them: Ohio, Florida, Virginia, Colorado, Iowa and New Hampshire.
National surveys show a majority Americans support legalizing gay marriage.
But most blacks — a core part of Obama's base — do not. And Obama needs them to turn out in huge numbers as they did four years ago in places like Cleveland, Richmond, Va., and Charlotte, N.C., in order to win in such battleground states. The same goes for Democratic-leaning Hispanic voters. Most of them oppose gay marriage, and their backing will be critical to Obama in cities such as Miami, Las Vegas and Denver.
At the same time, most evangelicals and other conservatives who make up the base of the GOP are strongly opposed to gay marriage. And Obama's position could end up unifying conservatives behind presumptive GOP nominee Mitt Romney, who has struggled to win them on his own. The former Massachusetts governor opposes gay marriage, but has spoken in favor of gay rights in the past.
It's an issue that motivates the GOP's conservative base to come out and vote, and that could have an impact in traditionally Republican states like Virginia and elsewhere.
North Carolina voters, for example, overwhelmingly passed a referendum Tuesday that strengthened the state's gay-marriage ban. It appears to have driven GOP turnout to record levels. Sixty-one percent of voters approved the measure in a traditionally Republican state Obama won four years ago.
A 2004 referendum to ban gay marriage in Ohio was credited with increasing turnout in the state's GOP-heavy south and west, helping George W. Bush win carry the pivotal state, though there's now debate over whether there actually was much of an impact.
That said, large majorities of Americans under 30 support gay unions, and Obama's move may fire them up enough to counteract any potential falloff by minorities and enthusiasm by conservatives.
Still, Democratic-leaning states like Michigan could become more attractive to Republican Mitt Romney and typically Republican-tilting states like North Carolina could become more perilous for the Democratic president, if the outcry on the right is great and backlash among minorities materializes.
Some doubt it will, saying that black and Hispanic voters will overlook their differences with Obama on gay marriage.
That may be the same for Hispanics, who will be pivotal to winning in the Southwest and in Florida.
"Social issues have never ever been an issue that they bring with them to the voting booth," said Maria Cardona, a Democratic strategist, who says Hispanics view the issue as one of civil rights. "Will there be one or two religious conservative Latinos who fall off? Of course, but they weren't going to support Obama anyway."
Nationwide and in presidential battlegrounds, the economy is certainly voters' top issue. A recent Gallup poll on the most important problems facing the nation showed gay rights cited by 1 percent or less, while 72 percent noted the economy.
With the economy vastly eclipsing other issues, even leaders in the fight against gay marriage see only minor impact in key states from Obama's pronouncement.
"Those who care about the issue, it perks up their ears up," said Iowa Republican Chuck Laudner, who fought efforts to legalize gay marriage in Iowa. "But it's not a game changer. It's never going to penetrate the price of gas."
Still, Obama's announcement only turned up the heat on an issue that long had been percolating in key states targeted by both the president and Romney.
None of the states expected to be hard-fought through Nov. 6 has a gay-marriage referendum on the November ballot, but nearly all have wrestled with the issue since the last presidential election.
This week, the issue is at center stage in Colorado, where the Democratic governor called the Legislature back into session this week to address a proposal to allow marriage benefits to same-sex couples. That state banned gay marriage in 2006.
In Iowa, opponents of gay marriage, who rallied in 2010 to oust three state Supreme Court judges over a gay marriage ruling, are focused on another judge facing retention and a state senator who has blocked a constitutional amendment.
Earlier this year, New Hampshire's Republican-controlled legislature failed to repeal the state law allowing gay marriage.
Florida voters overwhelmingly approved a constitutional amendment banning gay marriage in 2008.
Voters in Minnesota — considered a Democratic-leaning state where many expect Obama will win — will weigh in November a referendum to bolster the state's statutory ban on gay marriage with a constitutional amendment. It could become a more competitive state if gay-rights opponents use Obama's position to rally conservatives. The same could happen in Democratic-tilting Wisconsin, Michigan and Pennsylvania if the issue gains steam.