Glenn Greenwald asks how, given my skepticism that the president can move public opinion, I explain the change in the polling on gay marriage since President Obama’s comments.

Did President Obama change the nation’s mind on gay marriage? (Pete Souza/AP)

The poll Greenwald has in mind shows that there’s been an increase in support for gay marriage among African-Americans in Maryland. That is to say, among a group that tends to support the president, in a state that tends to support the president, Obama’s comments might have had some effect.

I think that’s actually consistent with my article, which argues that presidential persuasion doesn’t tend to move the nation, but does tend to polarize opinion among partisans.

What’s interesting about this particular case is that the issue doesn’t seem to be polarizing. Obama, by endorsing gay marriage, made it, in essence, the official position of the Democratic Party. But Republicans appear to have made a conscious decision to avoid making opposition to gay marriage the official position of the Republican Party. And because Obama isn’t proposing legislation to legalize gay marriage, there’s nothing forcing Republican politicians to take a stand. If Obama was proposing legislation, I think you would see more Republicans opposing him, and more polarization around the issue.

All that said, I don’t want to overstate my general position here. My article presents evidence suggesting that presidential persuasion rarely changes the nation’s opinions and can inadvertently make things worse by polarizing Congress -- which, when you need minority party cooperation to pass legislation, leads to gridlock. But I’d never say that it’s impossible for presidents to affect public opinion, much less the outcome of issues.

In particular, presidents are clearly agenda setters, and sometimes, by moving something to the front of the agenda, they can accelerate a shift that’s already underway in public opinion, or make voters think about an issue they would otherwise ignore. On gay marriage, for instance, the president’s endorsement might have led some in the African-American community to reevaluate their stance on the issue, and by opening that conversation with his own conversion, gave people who didn’t really care about gay marriage, or perhaps privately supported it, a kind of permission to admit they’d changed their mind on the subject.

More commentary on this subject:

Jon Bernstein: “So it’s not exactly that Obama influenced black opinions, would be my guess. It’s that African American voters who really don’t care very much one way or another about the marriage issue — but do consider themselves on Team Democrat — are now aware that marriage equality is the normal position of that team. Or, perhaps, that those who think of themselves (implicitly or explicitly) as Team Black now have a revised view of what that team’s position is. Or, perhaps, people who are on Team Church and Team Democrat now realize that those two are in conflict and they have to choose, while before they were getting only one signal.”

Note that Colin Powell now supports gay marriage, too. Here’s Adam Serwer: “Understand that exploiting the divide between socially conservative but religiously liberal minority groups and social liberals was the linchpin of [the National Organization for Marriage’s] strategy in Maryland, which is a very blue state with a large black population. NOM simply can’t win without winning black voters, and Obama may have made that impossible. Instead of black voters torpedoeing marriage equality in Maryland, as NOM had hoped, they now stand poised to secure it. “

Here’s more from John Sides on whether people have changed their minds , and more from Jon Bernstein on what it the theory.