From a Gallup poll released this morning:

For the first time in Gallup’s tracking of the issue, a majority of Americans (53%) believe same-sex marriage should be recognized by the law as valid, with the same rights as traditional marriages. The increase since last year came exclusively among political independents and Democrats. Republicans’ views did not change.

To put this into context, here is a graph that tracks public opinion since 1996:

Gallup poll