In the immediate sense, it was apparently the comments by Joe Biden (and to a secondary extent Education Secretary Arne Duncan) that forced the president's hand, leading to his historic announcement in support of same-sex marriage. But in the deeper and more long-lasting sense, the movement made him do it. That's exactly how politics on the left is supposed to work.
Franklin Roosevelt had the famous phrase: "Make me do it." He was speaking to activists for the labor movement or some other faction fighting for a slice of the pie, and he was saying to them, don't expect me to back you out of the kindess of my heart, even if in my heart I agree with you. This is politics, and you have to create the conditions that make it possible for me to support your cause. And that's what the LGBT movement did.
I think it's a remarkable--and as I've said several times, pretty risky--action on his part. As I wrote this morning, I think it's ballsier than Lyndon Johnson coming out for civil rights in 1964. Yes, I'm reading the criticisms--that he noted that he supports letting states decide. I guess it's a fair point to make, but honestly; it's not enough that a sitting president endorsed gay marriage? Really, that's the left for you.
Come on, people. This is a huge deal. This took cojones. But of course it'll take about five days before people on the left are back to grousing about him again. If that. But there's little doubt about what the history books will say. It's a great day for this country.