The significant increase in support for marriage equality is strong evidence that Presidents can change public opinion with advocacy, yes?To back up a bit...Nate Silver has the very tentative evidence that there has been a shift among African Americans; here's my contribution from Plum Line yesterday to what I suspect explains the shift, if it exists; and John Sides did an overview of it later yesterday and added some additional evidence about opinion leadership.
Which brings us to an excellent post by Scott Lemieux:
[A]ssuming arguendo that Obama’s position-taking has in fact increased support among African-Americans — this represents a fairly unusual political situation, in which 1)a stalwart part of the Democratic base 2)among which Obama is particularly popular has 3)a position that is in tension with much of the rest of the rest of the Democratic coalition 4)on a relatively low-priority issue for most voters 5)on which public opinion has been trending positively (including among African-Americans) anyway. It’s not like this kind of dramatic shift can be replicated in all that many other cases.Yes -- but there's more! The signal that's getting the response is an extremely strong one because it involves the president changing his position. That matters because it automatically made the president's advocacy front-page news; it also matters because anyone who wanted to keep her positions aligned with Obama's would, of course, have to switch.
The upshot of this is that if it is true, which we don't know yet, this is evidence of a situation in which a president may be able to affect public opinion, but it's one that isn't apt to show up very often at all or to be very useful. In particular, it doesn't apply at all to the most common situation, in which a president attempts to change public opinion by making a stronger, louder case for his already-declared policy position in order to sway people outside of his strongest supporters (who, usually, already agree with him).