If you were Mitt Romney right now, you'd probably feel like you're the victim of a double-standard. When you have changed your position on an issue in the past, everyone took it as proof that you have no core of beliefs and you'll flip-flop whenever the situation demands. But when Barack Obama does the same thing, he gets to say he has "evolved" and nobody takes it as proof of a character flaw. Surely, Mitt might be saying to himself, Americans will see this for the craven, politically motivated flip-flop it is and punish Obama for it, no matter what they think about gay marriage.
I'm afraid Mitt is going to be out of luck on this one. Obama's evolution will be treated differently than Romney's changes in position, for one important reason: because millions of people have gone through a similar evolution in the last few years.
Most of us haven't changed our opinions about abortion or cap and trade or gun control recently (if ever), but most Americans have changed the way they think about gay rights over the last decade. Don't forget that until about fifteen years or so ago, the question of whether gay people could get married wasn't what we call an "issue" because the idea was so radical virtually no one in our public debate was talking about it. In the time since, the average American has been forced to think about gay rights in increasingly complex ways. First they saw some gay characters on television. Then they learned that their cousin is gay. Then they found out that three of their co-workers are gay. Along the way they heard lots of public debate about gays serving in the military and getting married, and had to confront the narrowness of the arguments for the position they might have implicitly held before, though they would probably never have thought of it as holding a position. The more they thought about it, the less tenable the "it just doesn't seem right" argument seemed. Many public figures they respected, including many from both parties, became more accepting of gay rights. Even conservatives started talking about how much they opposed discrimination, even if they were still holding out against same-sex marriage (just one example: here's Dennis Prager, as hateful and demagogic a syndicated columnist as you'll find, writing about how he and other conservatives really aren't anti-gay and he even has a gay friend).
In other words, the overwhelming majority of Americans have "evolved" on the issue of gay rights (even liberals!). I'd wager that 90 percent of Americans think differently about who gay people are and what kind of equality they're entitled to than they did a decade or two ago. So the idea that Barack Obama did too isn't hard to understand. Of course, there are people who hate Obama and believe that everything he does is in service of a sinister agenda. But the median voter right now is someone who likes Obama personally, but thinks he has done only a passable job as president. That voter may not be a starry-eyed fan of the president, but s/he isn't inclined to think the worst of him either. So when he says "I've evolved," that voter says, "Yeah, I get it–I have too."
I'm not saying Obama's evolution wasn't entirely political. It probably was. We'll never know the contents of his mind, but the less charitable interpretation–that he has been privately pro-equality for a long time, but just didn't see the political advantage in coming out and say so until now when he got essentially backed into a corner–seems to me the most plausible one. But you know what? Who cares. Sometimes politicians do the right thing because they're principled, sometimes they do the right thing because doing so isn't much of a risk, and sometimes they do the right thing because they have no choice. In the long run, the effects on the country of a sitting president making this declaration are the same.