Support has been dropping at a rate of about a point year for the past 20 years. If that continues, it’ll sink below 50 percent sometime around 2024.

The good news? The big gay-marriage debate and probably even the big marijuana debate will almost certainly be long over by then, which means capital punishment will have center stage all to itself. Here’s your next social-issue hot potato, perhaps:

Follow the link to Gallup and examine the partisan split. Support has fallen across the board over the past 25 years, but the decline among Republicans (-9) is less than half of what it is among Democrats (-22) and independents (-21). Even so, both indies (60 percent) and Republicans (81 percent) are still strongly in favor, and even Democrats have a near majority (47 percent). I’m skeptical that we’re headed for a future where a majority opposes CP outright, but we may be headed for one where the margin is narrower than it’s been since the mid-60s.

Question: Why? What’s driving this? My hunch after first seeing the data was that stories about people being wrongly convicted for crimes or being disproportionately punished because of race was slowly shifting public opinion. And that may be so — but there’s nothing in the rest of Gallup’s data that suggests it. The number who believe the death penalty is applied fairly is a bit lower than it’s been in recent years but still a point higher than it was back in 2000; the number who say it’s applied too often is lower than it was as recently as 2004. No dramatic shifts in either metric.

My next guess, fueled by a tweet by the Guardian’s Harry Enten, was that this is really just a reaction to the long decline in America’s crime rate. In theory, the safer people feel, the less sharply they’re likely to react to violent crime. Whether rightly or wrongly, many people think capital punishment deters some murders that would otherwise be committed. If there are fewer murders happening, go figure that the clamor for CP diminishes. If you look at this graph, tracking violent crime in the U.S. over the last few decades, you’ll find corroborating evidence for Enten’s theory. Crime ticked upward in the early 90s before beginning to fall steadily circa 1994. Coincidentally, per the Gallup graph up top here, support for the death penalty maxes out in 1994 and then begins to drop. That’s the answer! Except for one thing:

That comes from a Gallup poll published in 2011 showing that lots of Americans don’t actually know that crime has been dropping over time. A Pew poll published in May of this year provides additional evidence. The number of crimes committed while using a gun has fallen along with the rest of the crime rate, but a clear majority of Americans (56 percent) thinks it’s gone up over the past 20 years. (Only 26 percent knew the truth.) How can support for capital punishment be eroding due to America becoming safer if Americans don’t know that it’s safer? When I asked Enten, he reasoned that Americans might personally feel safer even though they’re ignorant of conditions in the country at large, and that in turn might be driving their feelings about CP. He pointed to this metric specifically — fear of walking alone at night — as possibly a better barometer of perceptions of crime. I don’t know, though; here’s what the 2011 Gallup poll showed on that.

No slow and steady decline there since the mid-90s. In fact, the same Gallup poll showed a sharp sustained increase post-9/11 in the number of people who thought local crime, specifically, was increasing rather than decreasing. If being afraid for your personal safety fuels support for capital punishment, we shouldn’t be seeing a steady drop.

Exit question: So what’s the explanation, then? I’m tempted to say it’s simply a byproduct of liberals winning the culture war (gay marriage, legalizing marijuana), but that feels like a pat answer. You can be pro-SSM and pro-legalization while remaining firmly pro-CP for murderers (trust me), although maybe people’s tendencies towards political orthodoxy encourage them to treat culture-war issues as a package deal. Any theories?