I think a similar phenomenon is at work in both cases, as strange as that may sound, but I’m completely open to counterarguments. First the gay marriage numbers:

The most amazing thing to me about those trendlines is how steady they are across nearly every demographic. No matter how you slice the pie, with the important exception of religious groups, opposition drops about eight or nine points every four years; the only outliers are among Republicans, where it’s dropped 10 points total since 2004, and among blacks, where it dropped just four points between 2004 and 2008 and then a whopping 14 points from 2008 until now. While conventional wisdom holds that blacks are overwhelmingly opposed to gay marriage, the margin here between favorable and unfavorable has shrunk to just 10 points and the number who are strongly opposed is on par with whites. In fact, the only group that manages strong opposition above 40 percent is evangelicals, who are at 56 percent. (They’re also the only group that’s grown more strongly opposed over time.) I assume the broader population trend is due mainly to some gay-marriage opponents finding their worst fears of ill social effects unrealized as the practice has become more commonplace. That won’t sway all opponents — if you have a strong moral objection, you don’t need to reach the question of social effects to oppose gay marriage — but it’ll sway some. Note that even among seniors, opposition is creeping down towards 50 percent and strong opposition is already roughly on par with middle-aged adults.

Then there’s guns. Amazing:

That indie number will give you a good idea of why Democrats have stayed far, far away from this subject lately. This too is a policy matter on which black opinion is changing: 35 percent now say it’s more important to protect gun rights to own guns than to control gun ownership, more than double the number (17 percent) who said so in 2007. That puts blacks on par with women, 39 percent of whom say the same. (Really, ladies? Only 39 percent?) As for that astounding jump circa 2007, though, I’m eager to hear your explanations. My theory is that it’s a combination of the Heller litigation, which raised public awareness about the Second Amendment as a core American right, and increasing popular consciousness of the drop in crime rates over the last few decades. One of the key arguments for gun control is that the more freely available guns are, the more chaos there’s bound to be. Instead, empirically we’ve seen the opposite. Here too, then, as with gay marriage, you’ve got doubters who worried about ill social effects getting to see with their own eyes that the libertarian position isn’t nearly as dangerous in practice as they thought. Am I wrong about that? If so, what’s the alternative explanation? Media bias in favor of gay marriage is surely helping to nudge the numbers on that issue but I’m hard pressed to say that’s true for gun rights. Granted, Hollywood glorifies guns, but they’ve glorified guns since day one and only recently have we seen sharp movement in favor of gun rights in the polls.

Update: Ah, here’s an obvious alternative that should have occurred to me sooner: There’s a partisan effect on the gun question. Once you’ve got a liberal in the White House, Republicans and independents start to get nervous about gun-control measures being pushed and respond with a backlash. Not sure how that explains the rise in Democratic support during the first few years of O’s term, though. Maybe that’s rural Dems feeling the same fears?

Two other questions. If this is a partisan effect, why do the lines drop during Clinton’s presidency? And what’s with the jump after 2000? A reaction to 9/11 and terrorism, maybe?