The Upshot’s Nate Cohn has a nice item explaining why gerrymandering isn’t responsible for Republican majorities in the House (for more on this argument, see an old John Sides post). Dave Weigel has a good response, arguing that yes, gerrymandering is the culprit. Who is correct? They both are! But Cohn is, on the whole, more correct.

Cohn’s case follows what most political scientists have been saying: For the most part, deliberately drawn district lines aren’t the reason Democrats have received fewer seats in recent elections than the raw number of votes might indicate. Gerrymandering, by conventional measures, has cost Democrats only a handful of seats, not close to enough for them to have taken a House majority in 2012, when Democratic candidates received more total votes than Republicans. Instead, what’s hurting Democrats is “clumping” — Democrats are increasingly rolling up huge margins in small geographic areas. The result is a few House districts with overwhelming Democratic majorities, which in the language of districting means “wasted” votes for Democrats (wasted, because in simple plurality elections anything more than a single vote win “wastes” votes for the winning candidate that could be used more efficiently in other districts).

Weigel counters that there’s no requirement that lines be drawn the way they are. He shows that although overwhelmingly Democratic Philadelphia has been packed into as few districts as possible by gerrymandering Republicans, overwhelmingly Democratic Montgomery County, Maryland, has been sliced up by gerrymandering Democrats to help them get (relatively slimmer) majorities in a bunch of districts. After all, the only legal and constitutional requirements for district lines are equal population and the changing rules about ethnic dilution, neither of which forces the results Cohn and others consider the “natural” result of partisan clumping.1

Weigel is quite correct that there is nothing natural or necessary about keeping those dense groups of Democrats together.

There are, however, important political reasons the lines are drawn that way. Yes, one could slice up any large Democratic city, scattering its votes to many House districts. But cities don’t want to be split that way. Most local governments want House districts that respect both local government lines and communities of interest. What’s more, local politicians (who might want to run for those House seats some day) probably want House districts they have a chance of winning. And, perhaps most important, those city governments and their politicians are overwhelmingly Democratic, and they have unusual clout with Democratic state legislators who otherwise would be in a position to gerrymander.

That’s all straight-up interest politics, which make it difficult for Democrats to carve up cities for optimal partisan results.

It’s also the case that some good-government districting criteria tend to work against this particular type of gerrymandering. Good government groups (inexplicably) love straight lines and compact districts; carving up cities into pie wedges will probably draw their fire. And yet there’s nothing politically “neutral” about straight lines and rectangular districts; given population partisan distributions, it’s easy to read the political implications of that kind of district — and with Democratic clumping in the cities, those lines will favor Republicans in most cases.

So although Weigel is correct that there’s no technical reason that a dense Democratic city can’t be mined for votes with an effective gerrymander, the practical reality is that some gerrymanders are a lot harder than others. Or, to put it another way, if the definition of gerrymander is simply drawing district lines for political reasons, then the problem for Democrats is that the way populations are distributed makes it a lot harder for them to enact partisan gerrymanders. So it’s basically correct to say that Republicans have only netted a handful of seats from those partisan gerrymanders.

1 There’s also a requirement that districts must be contiguous, but that’s been loosely interpreted, with population centers connected by thin corridors.

[Cross-posted at Bloomberg View]

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Jonathan Bernstein is a political scientist who writes about American politics, especially the presidency, Congress, parties, and elections.