President Obama's endorsement of gay marriage undoubtedly reflects a personal evolution in his thinking, as he's said.

But his decision also reflects a hard-headed acknowledgement of the changing nature of the Democratic electoral coalition. Indeed, historians may someday view Obama's announcement Wednesday as a milestone in the evolution of his party's political strategy, because it shows the president and his campaign team are increasingly comfortable responding to the actual coalition that elects Democrats today-not the one that many in the party remember from their youth.

[AROUND NJ: VIEWS ON THE GAY STANCE]

Obama's senior advisers see the announcement as essentially a political wash, although polls now consistently show more Americans support than oppose gay marriage. In its latest national measure, the Pew Research Center found in April that a 47 percent to 43 percent plurality of Americans back same-sex marriage. Other recent national surveys, including those by Gallup and the NBC/Wall Street Journal poll, have found majority or plurality support for the idea.

Obama's announcement might not significantly change the overall level of his 2012 support, especially in an election where economic issues will dominate. But the announcement may reflect the Obama camp's thinking about the likely composition of his support. It shows the president, however reluctantly, formulating an agenda that implicitly acknowledges the party is unlikely to recreate the support it attracted from the white working-class and senior voters who anchored Franklin Roosevelt's New Deal coalition. Instead, the announcement shows him reaching out to mobilize the new pillars of the Democratic electorate, particularly younger people and socially liberal white collar whites.

(MAP: States That Allow Gays to Wed)

"It was crystal clear that he didn't want to get off the fence on this issue before the election if he could possibly avoid it; this is not a bright line he wants to draw," said long-term Democratic strategist Bill Galston, now a senior fellow at the Brookings Institution. "He realizes it intensifies the divide between what might be called the emerging Democratic coalition and the pieces of the old coalition that he wanted to retain. But things had gotten to a point where he felt he had no choice."

In one sense, Obama's shift on gay marriage does little more than catch him up to his party's mainstream; his ambiguity on the question risked leaving him behind a solidifying consensus within his party. By the end of this year, 13 of the 18 "blue wall" states that have voted Democratic in at least the past five consecutive elections may have authorized gay marriage or civil unions. Leading potential 2016 Democratic presidential contenders like governors Andrew Cuomo in New York and Martin O'Malley in Maryland have embraced the idea. And the most recent Gallup Poll showed 70 percent of Democrats backed same sex marriage.

(PICTURES: Evolution of Obama's Views on Gay Marriage)

Yet Obama's shift on gay marriage remains politically significant because many Democratic leaders have resisted embracing it for the same reasons the party has lost enthusiasm for gun control and blinked at fully pursuing comprehensive immigration reform. In each case, the party's primary fear has been losing support among the most socially-conservative elements of the white electorate: blue-collar, older and rural whites.

Those voters remain undeniably cool on gay marriage. In that most recent Pew Poll, while 47 percent of all Americans backed the idea, it drew support from just 40 percent of white men without a college education, 43 percent of white women without a college education, 42 percent of whites over 50, and a meager 32 percent of white seniors, according to figures provided by Pew. Those numbers help explain why virtually every Republican-leaning state in presidential politics, most of them in the South and across the Heartland, have banned gay marriage either by statute or constitutional amendment.

(RELATED: Demographics of Gay Marriage)

Yet even while resisting gay marriage, shelving immigration reform, and abandoning gun control, Democrats have hemorrhaged support among those socially-conservative white voters in recent years. In 2008, John McCain won nearly three-fifths of both non-college whites and white seniors. In 2010, the Republican House candidates won over three-fifths of the vote from both groups-and routed "blue dog" Democrats in rural and blue-collar House districts. Most 2012 polls show Obama struggling with both groups and in particular facing an uphill climb to match the mere 40 percent of the vote he captured among non-college whites last time.

Nothing about the gay marriage announcement will make it easier for Obama to recapture those voters. "I don't know that it makes it worse, but it confirms or is part of a narrative about white conservative voters not supporting him," says Democratic pollster Anna Greenberg. "These groups have basically decided they are not for him, and some of them decided before November 2008 and some decided during health care." Obama's willingness to publicly reverse his gay marriage stand may represent a tacit admission that he has to win without many of those older and blue-collar whites-and he needs to excite the constituencies more likely to actually support him in 2012.

The modern Democratic coalition has three principal pillars: young people, college-educated whites (especially women), and minorities. And with those groups, gay marriage is generally a much more popular position. In the Pew survey, half of college-educated white men, a resounding 65 percent of college-educated white women and 68 percent of whites under 30 backed the idea. (In 2001, just 52 percent of younger whites backed gay marriage.) African-Americans, Hispanics and other minorities are much more closely divided on gay marriage, with the Pew survey finding just 40 percent of non-white men backing the idea. But it attracts support from 54 percent of minority women and retains plurality support among minorities overall.(That itself represents a major increase from as recently as 2007 when only about one-third of minorities backed gay marriage in Pew polling.)

In an interview after the 2010 election, David Axelrod, Obama's senior campaign strategist, cited as a model for the president's 2012 coalition, the electoral alignment that narrowly reelected Democratic Sen. Michael Bennet in Colorado during the GOP mid-term sweep. Bennet won despite a stampede toward the GOP among blue-collar and rural whites by mobilizing young people, minorities and socially liberal upscale whites, especially suburban women (he carried 60 percent of college-educated women according to exit polls).

Economic issues will unquestionably matter most in November. But Obama's embrace of gay marriage, like his more enthusiastic recent endorsement of comprehensive reform that provides a pathway to citizenship for illegal immigrants, may make it harder for him to hold states like North Carolina, Ohio and even Wisconsin, where the most socially conservative elements of the white electorate are heavily represented. (North Carolina with the big evangelical Christian population that powered approval of Tuesday's state referendum to ban gay marriage appears to be an especially big concern for Obama's team.) Yet these positions could also improve Obama's prospects of assembling a Bennet-like coalition in states defined by high education levels and diversity like Colorado, Virginia and even Pennsylvania, which Mitt Romney is targeting.

"This is the clearest possible tilt toward what I would call the Colorado option [in the electoral map]," said Galston, a former deputy issues director in Bill Clinton's White House who has questioned whether Democrats can win without holding Midwestern blue-collar battlegrounds. "Colorado is now clearly the image of the Democratic Party's future and it remains to be seen whether what's a majority in Colorado will be a majority in the country."

 At the least, Obama's stances on gay marriage and immigration will have the effect of accelerating the transformation of the Democratic coalition from what it was to what it is becoming. For the president and his team, this wasn't quite a moment of burning the bridge behind him to the New Deal coalition-but it's close.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.