In Gallup’s polling, support for same-sex marriage rose from 35 percent to 42 percent between 1999 and 2004, but then dropped back to 37 percent; it rose to 46 percent just before Obama’s 2008 victory, but then dropped back to 40 percent a year later. Today’s 50 percent support likewise represents a slight drop-off from the high of 53 percent in the survey Gallup conducted last year.

This pattern suggests that Americans grow more resistant to same-sex marriage the more they feel that it’s being imposed upon them by an unelected judicial elite, and grow more supportive the more it seems to be gaining ground organically. A president is not an unelected judge, but a public flip-flop on the issue by the nation’s chief executive might feel like yet another elite attempt to pre-empt a debate that appears to be moving toward a resolution, but hasn’t quite been settled yet.

The second reason for the White House’s caution is that opinion polling has consistently understated opposition to same-sex marriage since the issue rose to national prominence. Voters who say they support it when Gallup and other pollsters come calling can behave very differently in the privacy of the voting booth.