Tuesday, December 18, 2018

The Impact of Gerrymandering, Visualized

In 2012, Democrats were outraged at the gerrymandered congressional map. They won the national popular vote by 2.5 million votes (once you account for uncontested races), but lost the house 234-201.

This time, there hasn’t been as much outrage, because Democrats managed to win despite the map. Some people on the right have been saying that the congressional map was in fact fair this election, because Democrats’ share of seats approximately equaled their share of votes. That’s really misleading.

The congressional map remains plenty biased. Democrats won 235 seats and 6.7% more votes (more on how I arrive at 6.7% later). Republicans could have won 235 seats with a margin of just 1.1% more votes. That 5.6% difference is what Democrats needed to overcome the congressional map. Talk about doing it backwards and in high heels!

The following graphs show how the 2018 election would have gone under various popular vote scenarios. To model a popular vote tie (0% margin), for example, I shifted each district to Republicans by 6.7%. There were 204 districts that voted for Democrats by more than 6.7%. Democrats would have held those under a popular vote tie, while the other 231 would have gone to Republicans.

Majority (218) Proportional Republicans Democrats 235 235 255 255 0 100 200 300 400 Seats -100 -50 0 50 +100%  Popular Vote Margin

The S-shape is from the winner-take-all districts. Because we have winner-take-all district, a slight edge in the national vote total can translate into a big edge in seats—the winning party wins a lot of close elections. But winner-take-all doesn’t have to be unfair to one side or the other. In the graph above, if the parties’ lines were on top of each other, it would be fair. The gap between the lines is the distortion caused by the map.

Democratic voters are packed into heavily Democratic districts, so they’re guaranteed a larger base. If Republicans won overwhelmingly, winning the national popular vote 75%-25%, they’d still lose 34 seats. Those 34 seats are Democrats’ base. The 34 districts are in cities, including 9 in New York City alone, and 31 of the 34 are majority minority.

On the other hand, if Democrats won 75-25, they’d win all but 12 seats. Democrats would sweep Louisiana, for example, where there are 5 districts that are currently 70% Republican. So Republicans’ base of guaranteed seats is smaller.

But it doesn’t help to have more seats you’re guaranteed to win; the goal is to win a majority. Running up big margins in your strongest districts doesn’t help. And because Democrats are packed in their blue islands, swing districts are more conservative than the nation as a whole.

For the foreseeable future, no party is going to win by more than 20%. So let’s zoom into the above graph, showing only elections that are 60-40 or closer.

235 235 255 255 Majority (218) Proportional 200 300 Seats -20 0 +20% Popular Vote Margin

We would have had a repeat of 2012 if Democrats had again won by 2%: Democrats need to win by 2.7% (i.e. win 51.4-48.6) to get a majority. Republicans can win 218 seats, a majority, if they lose the popular vote by 2.7% or less. That’s a gap of 5.8 percentage points! For this year’s total of 235 seats, as I mentioned earlier, the gap was 5.6%. I calculated the gap for the average election, using past vote totals. On average, it’s 5.2%. This means that 5.2% of the electorate voted for Democrats and their votes were cancelled out by the map.

Gerrymandering or Geography


So far, I haven’t said anything about whether this gap is driven by gerrymandering (Republicans drawing districts that pack Democrats together) or geography (Democrats live around other Democrats). Both of those could explain why swing districts lean Republican. To answer this question, we need to know what a non-gerrymandered map would look like. But how do you define that?

Dave Wasserman at FiveThirtyEight drew hypothetical congressional maps by following a few different rules. One rule was to make districts as compact as possible. That results in hilarious-looking circular districts. Another, more plausible, rule was to make districts as compact as possible while trying to follow county borders. Here’s Ohio as an example.

So how fair are these un-gerrymandered, compact districts?

Actual Districts Compact Districts 249 235 235 255 255 100 200 300 Seats -20 0 +20%   Popular Vote Margin

They’re better, but they’re not fair. Democrats need to win by 1.9% to get a majority, instead of 2.7%. The gap between what Democrats need and what Republicans need—the wasted votes—is 3.6% on average, instead of 5.2%. So about one third of the bias in the map is gerrymandering. Compact districts are an improvement, but those lines aren’t that much closer together. Two thirds of the gap is geographic reality.

Geographic reality doesn’t mean Democrats are helpless. The courts could require states to draw districts to maximize partisan fairness, not to be compact. There’s no inherent virtue in compactness.

On the left is FiveThirtyEight’s attempt to make each state’s congressional delegation proportional to its vote. On the right is their attempt to make as many swing districts as possible.



Those look a lot fairer (slightly favorable to Democrats in spots, thanks to FiveThirtyEight’s drawing). Courts have emphasized compactness when ruling on redistricting cases. But they could shift, and focus on partisan fairness. Democrats could gain some ground if they just fight for compactness, but unless they fight for partisan fairness, the map will still be biased against them.

There are also some more radical options for Democrats. If the Uniform Congressional Districts Act were repealed, states could shift to multi-member districts (which existed in the early 1800’s), or other voting systems entirely, like mixed-member proportional. So I don’t think we should take it as a fait accompli that the congressional map will hurt Democrats just because they’re clustered in cities.

Nonetheless, when people talk about gerrymandering, they tend to mean splitting counties and drawing crazy lines for partisan gain. No one would look at those compact maps of Ohio I showed and say: “those look really gerrymandered” even if they do, in fact, pack Democrats together. So while compactness isn’t fair, it is the norm.

To measure gerrymandering, then, I’ll compare each state’s current map to the compact maps. I’d call this malicious partisan gerrymandering, as opposed to geographic bias. Democrats won 235 seats under the current map, but win 249 under the compact map. Here’s which states are responsible.

Texas, Ohio, Florida, and North Carolina are known to be the worst offenders, so it’s good to see them at the top of this list. On the other side, California’s districts were drawn by an independent redistricting commission. But the commission drew competitive districts, and in the ensuing wave, Democrats swept them.

Below are the results for six different maps that FiveThirtyEight drew.



You can see that the round compact districts and the county-based compact districts produce almost identical results. Even though they look very different, they both say that gerrymandering is responsible for a third of the gap between the parties. And both compact maps say that Democrats are hurt most in landslide elections like this one. That could suggest that gerrymandering protects Republican incumbents more than it adds new Republican seats.

Getting the Right Total for the Popular Vote

So, about that 6.7% figure for the popular vote. Democrats won 53.4% of the vote, while Republicans won 44.8%. That’s an 8.6% margin of victory. But there were 41 uncontested races, and 38 of those were Democrats without a Republican challenger. Instead of giving the winner 100% of the vote in those districts, I filled in the results by assuming they swung the same amount from 2016 as other districts (actually the same amount after a log odds transformation, which is how I dealt with all the popular vote swings). That yields 6.7%.

Now some people have been comparing the 8.6% figure against Republicans’ 2010 margin of 6.8%, or their 1994 margin of...also 6.8%. Was Democrats’ 2018 wave actually smaller than those Republican waves? No, because in 1994 and 2010, 30 and 24 Republicans ran unopposed (compared to 14 and 5 Democrats—sensing a pattern?) The Economist made an effort to correct for this, and while I don’t know their method, they got adjusted margins of about 5% for each of those Republican wave years. Here’s their graphic with 6.7% for 2018 added.

In the 2016 Presidential and the 2012 House elections, Democrats won the popular vote by 2% and lost the election. Republicans won 7 more seats 2010 than Democrats won this year, despite a smaller popular vote margin. The bias in the system is here to stay. So tune out anyone who says Democrats’ victory wasn’t convincing—no one said that in 2010.

The code and data are available on my Github, and you can find more thoughts on data, politics, and economics from me on Twitter. Credit to Dave Wasserman, whose popular vote tracker and district maps were indispensable.

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