The efficiency gap also adds votes for losing candidates to the “wasted voted” pile. In this case, 80 Democratic votes were wasted in the two races Republicans won (40 in each case), for a total of 99 wasted votes. Republicans, meanwhile, only wasted 30 votes in the race they lost to the Democrat, giving them a final tally of 48 wasted votes. Subtracting the GOP wasted votes from the Democratic total gives us a remainder of 51 votes. Remembering that there were 300 votes overall, 51 divided by 300 equals an efficiency gap of 17 percent, marking a very large penalty against Democrats.
In our hypothetical example, one party easily wins a two-thirds majority of seats even though the statewide support for both parties was exactly the same—an ideal outcome for anyone wishing to put in place a partisan gerrymander, but not a positive result for democracy. The creators of the efficiency gap contend that a gap of 7 percent or more in favor of one party is so historically atypical that it amounts to evidence of a partisan disparity so extreme as to potentially be unconstitutional if there aren’t mitigating factors like the state’s underlying political geography.
The Brennan report also explores two other tests. One, called the “seats-to-votes curve,” compares the share of seats a party wins with its statewide share of the two-party vote over time to create a graph based on historical data. If a party consistently wins far fewer seats than its statewide popular vote share, particularly if the popular vote is relatively evenly divided, it’s a sign that the map significantly favors the other party. And if such a curve is relatively neutral between the parties historically but a new map produces a result that’s far out of line with past expectations, it could be a sign of nascent gerrymandering.
The third approach is known as the “mean-median” test, which uses a statewide election instead of legislative or congressional results. As Daily Kos Elections has previously outlined in this explainer post, this test would, for example, rank every district from Hillary Clinton’s biggest victory margin to Trump’s biggest victory margin. The district that falls in the middle of that ranking is thus the median district. In our approach, which is similar to Brennan’s, we compare that median district to the overall statewide margin, which can reveal a partisan edge. For example, even though Clinton won Virginia by more than five points, the median seat favored Trump by more than three, giving the GOP an almost nine-point advantage.
Applying these three tests, the Brennan Center finds that Republican-drawn states have by far the largest partisan advantages, particularly in heavily populated states like Florida, Michigan, North Carolina, Ohio, Pennsylvania, Texas, and Virginia. By contrast, the rare Democratic-drawn states such as Maryland or Illinois have much smaller or even negligible Democratic advantages. All three tests have their drawbacks, which the report itself details, but the totality of evidence leads to only one conclusion: Republicans enjoy a significant partisan advantage thanks to the way they have drawn congressional maps across the country.
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