Obama, after all, had good reasons to resist this shift up to now. Those had everything to do with his re-election, rather than his personal feelings on a matter that many Americans (and presumably the president) have regarded for some time as a civil right.
The move is likely to hurt him in the South. One in three Southern swing voters are strongly opposed to gay marriage, a recent Pew Research Center poll found. Just this week, North Carolina, which Obama carried narrowly in 2008, approved one of the toughest bans on same-sex unions in the country. If the switch on same-sex marriage ends up costing him states in the fall, North Carolina will be the first place to look.
More central to his re-election will be the effect in Virginia, one of the top battlegrounds in the country, where a recent statewide survey showed him with a slight lead over Mitt Romney. "There could be some element of risk there," said veteran pollster Andrew Kohut, who directs the Pew Research Center.
The reactions of minority voters will be closely watched. Blacks are unlikely to abandon the president, though his decision will surely give some older blacks pause. Six months of relentless assaults by the Romney forces, however, will likely boost their determination to see Obama re-elected.
Some Hispanics may well have second thoughts, particularly voters cross-pressured by the Catholic church's aggressive opposition to gay marriage.
"Hispanics who are less committed to Obama and are otherwise upset because they feel he broke his promise to pass immigration reform in his first year in office might have one other reason to give (Romney) a look," said Whit Ayres, a Republican pollster. Privately, some Democratic strategists share that view.
Up to now, social issues were expected to play a minor role in a presidential election that seems all about the economy and jobs. A downscale white voter in Ohio who was already leaning against the president might lean a little bit more against him now, but it's unlikely that the gay-marriage decision, alone, will sway a large number of swing voters.
But if the election is close, small things will add up. Obama was forced into accelerating a step he preferred to avoid after Joe Biden blurted out his support for same-sex marriage on a weekend talk show. The vice president's departure from the company line only served to make Obama's reluctance look more calculated -- at further cost to his already badly tarnished reputation as a different kind of politician.
"The more he tried to explain 'evolving' the less sincere it seemed," said Celinda Lake, a Democratic pollster. She called Obama's shift "a sign of leadership and authenticity" to the liberal base and younger voters, in particular.
Where the issue will resound loudest is among those who feel the strongest on gay marriage, pro and con.
Conservative Christian strategist Ralph Reed described Obama's decision as a "flip-flop" and "an unanticipated gift to the Romney campaign. It is certain to fuel a record turnout of voters of faith to the polls this November."
Obama's decision unleashed fresh excitement among gay and lesbian donors, already one of the largest sources of campaign money for the president's re-election.
"Today has focused attention and enthusiasm in an almost cathartic way," said Andrew Tobias, treasurer of the Democratic National Committee and a top gay bundler for Obama. "Within minutes, people were calling with their credit cards. They're thrilled."
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