Many enviros have criticized Obama for not using the "bully pulpit" adequately to highlight the dangers of climate change. Other activists make similar claims for their causes. By contrast, most political analysts seem to think the bully pulpit doesn't actually work.
I think there's a measurement problem - short term changes could be noise or ephemeral reaction to the bully pulpit with no lasting implications. Long term changes can't distinguish between bully pulpit causes and everything else in life. My personal guess is that it plays a modest but not insignificant role in cultural attitudes, and plays a somewhat more important role in elite attitudes.
Obama's shift on same sex marriage provides the best hope that the bully pulpit can actually do something. Public opinion has shifted dramatically on marriage equality in the last 20 years with some evidence of acceleration, so if there were ever a chance to accelerate it further, now seems like a great time.
Most polls I've seen following Obama's announcement have shown large shifts in African American public opinion (but not all polls) and little shift in non-black opinion. Some analysts have complained that this shift just reflects people lining up with their side, as if that matters. Anyway, it's short term. Long term, we all know that in a decade support for SSM will be the dominant position outside of the South but there would be no way to test whether the extent of the dominance was affected by Obama.
Let's look in the medium term, one or two years from Obama's announcement. My guess, made before Obama evolved, was that his opinion could shift 5-7% of black opinion and 1-3% of non-black opinion. I think I'll stick with that and look at polls in 2013. The complicating factor is that support for SSM typically goes down in presidential election years as the Republican Party plays the anti-gay marriage card. Might not happen this time, but who knows.
My prediction is that we'll see the biggest bump in African American support in 10 years, big enough to overcome any Republican push against marriage.
Moving past the medium term, I think the biggest effect of Obama's announcement is to line up the Democratic Party in all the same direction. Democratic leaders in blue constituencies will find it very hard to resist supporting gay marriage. Here in my area, the otherwise popular mayor of San Jose is finding that out, something that could really hurt his political future. Democratic leaders in red constituencies and among African Americans now have much more room to support SSM. This probably falls into the long-term category, though.
I'm not sure how this all plays into the debate over whether Obama should talk more about climate. Overall I think that yes he can and yes he should, but his political actions are more important than the small effect he can have on public opinion in the medium term.
UPDATE: found a prediction I made in 2005.
The actual timeline is that they'll benefit politically for another five years, it won't help or harm them much overall for ten years past that, and then for fifteen years it'll hurt them as they fight the fundamentalist dinosaurs in their party before they can finally get rid of the anti-gay marriage plank of the Republican Party platform.I was too pessimistic overall. Beginning in 2014, being anti-SSM will start hurting Republicans on the national level (this year it's neutral). It won't take Republicans 30 years to drop opposition, more like 15-20 years.