With Veepstakes heating up — Mitt Romney’s campaign is now officially in the process of vetting potential running mates — it’s time for a quick refresher on how the bottom of the ticket affects November outcomes.

Quick version? It doesn’t, much.

Okay, that’s a little glib. But you can probably safely ignore anything about the chemistry between the candidates, or how a pick contributes to the campaign “narrative,” or anything about particular demographic groups that may or may not be attracted to particular selections. There’s very little evidence that any of that matters in any measurable way.

Indeed, political scientists who have studied the issue have mostly found little effect. Vice presidential picks can help the ticket a little bit (a couple of percentage points, maybe) in their home state. That’s not nothing! If Rob Portman can swing Ohio or Marco Rubio can give Romney Florida in an otherwise very close election, that could be decisive, so I wouldn’t dismiss it. But it’s a very limited effect.

The downside appears to be limited, too, although a real dud pick, as Sarah Palin was in 2008, appears to be able to push the national vote a couple of points.

The rest of it, however? Just a lot of hype. The main reason that VP selections matter is that an awful lot of them wind up as presidents or presidential nominees. As far as the current election goes, however, don’t expect much. Party matters to voters, and so does their evaluation of the incumbent. Campaigns in general, and the presidential candidates, matter a lot less; by the time you get to running mates, there’s not a lot of decision left to be made. So enjoy Veepstakes for the diversion it is — hey, it’s better than the phony outrage and other one-day controversies that the cable networks generate and feed on — but don’t expect it to make a big difference in November.