Obama supports gay marriage: How will it affect his re-election campaign?

— -- President Barack Obama's history-making embrace of gay marriage could send far-reaching political aftershocks through a presidential campaign defined by voter concerns about the economy but likely decided by slivers of the electorate in a handful of battleground states.

"The politics, it's not clear how they cut in some places that are going to be pretty important in this electoral map. It may hurt me," Obama told Robin Roberts of ABC News in an exclusive interview Wednesday as he announced his change of heart.

A Democratic senator who supports Obama's re-election as well as gay marriage told Yahoo News that the president could face "significant electoral risk" if his announcement is merely a check-the-box exercise with no follow-through.

"If the president simply makes an important commitment to equality and moves on, and does not challenge the network of people nationally, activists and others, who favor marriage equality, to speak up from now through the election, he is taking a significant electoral risk," said the senator, who spoke on the condition that he not be identified by name.

"When you move from 'civil union' to the word 'marriage,' it reaches people differently," the senator said in a telephone interview, warning that highly motivated foes of gay marriage will unleash a campaign of "public speeches, sermons, newsletters, websites, that will darkly suggest a negative future."

Obama echoed that sentiment, telling ABC News: "I had hesitated on gay marriage in part because I thought civil unions would be sufficient, that that was something that would give people hospital visitation rights and other elements that we take for granted and and I was sensitive to the fact that for a lot of people, you know, the word marriage was something that evokes very powerful traditions, religious beliefs, and so forth."

Richard Socarides, a leading gay activist who served as President Bill Clinton's top adviser on issues like same-sex marriage, said Obama "can help build a national consensus."

"Nobody expects the president to make this a central feature of his presidency, but it is an important issue," Socarides told Yahoo News by telephone.

Other Obama supporters pointed to a Gallup national poll, released Tuesday, showing that the country has been "evolving" on the issue along with the president.

The public opposed gay marriage by a lopsided margin of 68 percent to 27 percent when Gallup first asked the question in 1996. In 2012, for just the second year, a narrow majority of 50 percent favored making it legal, with 48 percent against. (The margin of error was plus or minus 3 percentage points.) Independent voters were strongly in favor, 57 percent to 40 percent, which on the surface would seem to help the embattled incumbent.

In what both sides expect to be a hotly contested election, the outcome could turn on relatively few voters in up-for-grabs states--like North Carolina, which voted 61 percent to 39 percent Tuesday to adopt a constitutional amendment banning gay marriage and civil unions. Obama beat John McCain there by just 13,692 ballots in 2008, and he leads Mitt Romney by 2.4 percentage points according to a Real Clear Politics average of polls.