First, let’s look at the existence of Republican gerrymanders. It is rather smirk-inducing, but there are still plenty of respectable folks in the community of electoral pundits who basically deny the existence of Republican gerrymanders altogether.

There is an operant logic to this, of course. It is a theory which rests on the impact of something called geographic sorting. For those who are new to the concept, it is the tendency of Democratic voters (particularly African-American and Latino voters) to cluster tightly in urban/suburban neighborhoods, thus making electoral districts in those locales excessively partisan. Republicans, by virtue, are not clustered quite as tightly, thus giving them a nominal advantage in district-based elections, even if the Democrats score a higher net proportion of the votes for that office (be it federal or state).

Back in 2013, I explored that question, and the conclusion was pretty ambivalent. Yes, it is without question that geographic sorting is a real thing, and it explains why Democrats can very plausibly carry the “overall” popular vote for a state legislative body (or the U.S. House of Representatives) and yet remain in the minority. Which is why Democratic partisans who cry foul that the Democrats logged one million more votes in 2012 in the national popular vote for the U.S. House of Representatives are, to an extent, missing the point. If we had some kind of an at-large representative system (or proportional representation), that would be a relevant statistic. But when our house is determined by 435 individual elections (and our state legislatures, with a few exceptions, are determined by several thousand individual elections), the “overall vote” doesn’t really matter all that much.

All that said, geographic sorting does not, in itself, explain the current disparities at the federal or state levels. Gerrymandering is a real thing, and it has distorted the outcomes of the federal and state delegations in many states in our union. At the state legislative level (which is our area of focus over these past three articles), it has had profound consequences, not only in leading the Democrats to record-low numbers of chamber majorities but also in leading to an outsized number of races where one of the major parties does not even bother to field a candidate.

One way to measure the extent of a gerrymander is to look at the “median district” in a given state. In theory, in a “fairly drawn” state, the median district should be reasonably close to the state’s overall presidential performance.

For example, let’s take a state whose districts have long been drawn by an impartial commission: Iowa. In the Iowa state senate, the median district, in terms of 2012 presidential performance, would come in at 52.6 percent Obama. This is actually marginally better than the president’s 2012 statewide performance (Obama won 52.0 percent). But Iowa is a special beast—the rather homogeneous population there makes geographic sorting less likely to be of consequence than in a state like Michigan or Ohio. The Iowa state house of representatives, meanwhile, is a touch more conservative than the state at-large, coming in at a median district which would be 51.3 percent Obama.

Now, let’s look at a state that is pretty universally accepted as a Republican gerrymander: North Carolina. Admittedly, Mitt Romney won North Carolina, but recall that he did so with a rather modest 50.4 percent of the vote. The median state senate district in the Tar Heel State went for Mitt Romney by … 57.1 percent of the vote. The median state house district went for Romney by an only slightly more modest 55.9 percent of the vote.

So, by way of comparison—both chambers of the Iowa state legislature have median districts which are within seven-tenths of a percentage point of the actual state presidential performance. Both of the chambers of the North Carolina state legislature have median districts which are more than five points more Republican than the state was at-large in 2012.

Now, the “it’s all geography” crowd would undoubtedly note that North Carolina has a more diverse population, and therefore would be far more likely to be clustered tightly. But a five to seven point disparity between the median district and the actual performance is hard to justify as normal clustering.

Take a diverse state that did not have a Republican gerrymander: Colorado. Colorado had a redistricting process where a commission draws the maps, and the state court approves the map. The median state house district was actually a few points more Democratic than the state at-large (55.2 percent, whereas Obama won 51.5 percent statewide). The median state senate district was almost dead-on with the state average, at 52.1 percent Obama.

Even states where the Democrats did draw the maps do not see the outsized disparities we witness in North Carolina. Take, for example, Illinois. Illinois’ median state house district comes in at 56.7 percent Obama, which actually gives the lower house a slightly R-leaning median (Obama carried his home state with 57.5 percent of the vote. The Senate was virtually identical (56.8 percent). In other words, if the goal of the Illinois map was to screw the Republicans, it didn’t do a particularly good job of it.

Contrast that with Illinois’ neighbor to the north—Wisconsin. Wisconsin’s state senate has a median district (an open seat in 2016, by the way) that Mitt Romney carried with 50.0 percent of the vote. The median state house seat leans a bit more Republican, at a 50.8 percent Romney share of the vote. Romney, you will recall, lost the state of Wisconsin with 45.9 percent of the vote. So, the tilt is not as extreme as North Carolina, but it is fairly pronounced.

But the key to an effective gerrymander isn’t just to tilt the median—it is to pack the legislative body with a lot of races your party can be expected to win, albeit marginally. Here, Wisconsin is an incredibly apt example. Badger State Republicans managed to massage the map in order to make sure that 56 of the state’s 99 state house districts were carried by Mitt Romney, despite his losing the state by seven percentage points.

What this means, of course, is that Wisconsin Democrats have to over-perform one of their better recent electoral cycles, and by quite a bit, in order to have a shot at the majority.

A similar scenario exists across Lake Michigan. In the state of Michigan, a state that Republicans have not managed to win in a presidential election since 1988, the Republicans managed to cleverly draw their way to a map where Mitt Romney carried 57 seats to Barack Obama’s 53. This is despite the fact that Obama won the state 54-45. Or, to use a different metric, Romney outpaced his state performance (44.6 percent) in 69 of the state’s 110 state house districts.

Immediately, I can almost hear the bleats of the “it’s not gerrymandering” crowd. And they are all saying the same word: Detroit. Without question, if there is a state that’d have a model example of clustering, it’d be Michigan, what with the sheer volume of Democratic support lying within a few dozen miles of the site of Netroots Nation 2014.

However, it must be said: It’s not like voters of color suddenly coalesced in the greater Detroit area sometime in the last five years. This, in essence, is the most readily identifiable flaw of the “it’s all clustering” argument. The kind of de facto segregation that creates these heavily Democratic enclaves didn’t somehow emerge from the ether since Obama came into office. Those communities have been heavily of color for decades, of course.

Also, and this should be evident to any seasoned electoral observer, Democrats were not only competitive at the state legislative level less than 10 years ago—they were thriving. Michigan is actually an intriguing example. The state senate has always been just out of reach, probably because the larger district sizes make the clustering effect a little more pronounced. But the Michigan state house was highly competitive for the previous two decades before 2010, and actually tilted markedly Democratic in 2008. It was only after the Michigan Republicans hit the legislative trifecta in a crushing statewide landslide in 2010 that the map turned so darkly, and impenetrably, red.

This 2010-inspired shift to the GOP, fueled by opportunistic cartography, is the hill that downballot Democrats now have to climb. It’s not enough just to beat the GOP. It’s not even enough to cover the kind of spread that “geographic sorting” might generate. They must have a hurricane-force wind at their back. Democrats have to both hope, and work their asses off, to ensure that one of the next three election cycles creates that kind of gust. Absent that, they could be shut out of the redistricting process again at the start of the next decade.

That realization is not lost on the campaign arm of state legislative Democrats. The Democratic Legislative Campaign Committee (DLCC) has already launched a coordinated effort, which they call Advantage 2020. It may well be the most important campaign effort that many Democrats and political junkies haven’t heard of.