President Obama's historic decision to endorse gay marriage has not only stoked the enthusiasm of his liberal supporters — it has fired up his opposition, too. Obama's declaration seems sure to spur many social conservatives to set aside their doubts about Mitt Romney and rally behind the presumptive Republican nominee, and could lose the Democrat the votes of some independents, too. That complicates Obama's effort to win in several swing states, many of which have passed referendums banning same-sex marriage. Indeed, says Charlie Mahtesian at Politico, "for all the polls showing movement toward greater public acceptance of gay marriage, for all the signs of increased tolerance and changing mores, there's one undeniable fact: A full embrace of gay rights has never been a winner in the political arena." Which states might punish Obama for his evolution? Here are four:

1. North Carolina
Obama's decision could cost him in plenty of states, says Amie Parnes at The Hill, but "it's North Carolina that poses the biggest risk for the president." Sixty-one percent of North Carolina voters just backed an amendment to their state's constitution banning not just gay marriage, but also civil unions. Obama narrowly won North Carolina in 2008, and he was spared a loss in the Tar Heel State thanks to heavy turnout among African Americans, many of whom oppose gay marriage. Most black voters in North Carolina are expected to stick with Obama, but "losing some, or depressing African American turnout even slightly, could cost the president" dearly in a tight race.

2. Colorado
In recent years, Colorado voters have voted against gay marriage and civil unions, and Republicans who control the state legislature recently blocked a new measure that would have recognized civil unions. The state's No. 2 city, Colorado Springs, has even been called the nation's evangelical capital. As a result, Floyd Ciruli, a Colorado-based pollster and analyst, tells NPR that Obama's gay-marriage stance could make the race so close that it's decided by "a couple of percentage points." Obama might get a bump in turnout from his base, but "up to a third of Democrats and 40 percent of unaffiliated voters are opposed to gay marriage," so it's just as likely that this could put Romney over the top.

3. Florida
If gay marriage does one day become law across the nation, says Charles Mahtesian at Politico, "it won't be until after the current generation of senior citizens passes away." They overwhelmingly oppose it, and they vote in heavier numbers than other age groups. And nowhere are older voters more of a force than in Florida, which also happens to be a critical swing state with the kind of electoral clout that tips national elections. In 2008, "Obama and an anti-gay marriage constitutional amendment shared the Florida ballot. Obama won the state narrowly, [but] the amendment won by a landslide." 

4. Ohio
A referendum to ban gay marriage passed in Ohio in 2004, and has been credited with fueling conservative turnout, and thus helping George W. Bush beat Democrat John Kerry in the state, says Mahtesian. "It's an issue that resonates outside of Democratic vote centers like Columbus and Cleveland," and political strategists say opposition to same-sex marriage significantly helps Republican candidates in Ohio's southern Bible Belt area. And remember: Ohio has backed the winner in the last 10 presidential elections.