North Carolina Republican state Sens. Dan Soucek, left, and Brent Jackson review historical redistricting maps. (Corey Lowenstein/The News & Observer via AP)

The Washington Post's Michael Scherer has a must-read piece today about a Justice Department push to include a citizenship question on the 2020 U.S. Census. While that might seem like the arcane workings of the federal bureaucracy, it is a decision that carries potentially major political ramifications — most notably for Republicans' ability to gerrymander Democrats into the minority for years to come.

Here is why: Republicans already have a significant edge on the congressional and state legislative maps, thanks to how our population is distributed and to the GOP having earned the power to redraw lots of the new maps after the 2010 Census. This could significantly increase their advantage for two reasons:

  1. It might dissuade noncitizens from participating in the Census, thereby diluting the political power of the (mostly urban and Democratic) areas they come from. And . . .
  2. Even without that, it would hand Republicans a new tool in redrawing districts even more in their favor.

This has been percolating for a while, ever since the U.S. Supreme Court's decision to take up a case called Evenwel v. Abbott. In its 2016 ruling on the case, the court unanimously ruled the “one person, one vote” standard did not require states to draw legislative districts according to voting-eligible population — that is, without including noncitizens and children.

That was a win for Democrats. In the ruling, the Supreme Court did not prohibit states from drawing state legislative districts by that standard, and Justice Samuel Alito suggested that would be a future question for the court to decide.

To be precise, federal courts have long ruled congressional districts must use total population, so this is not about them — at least not directly. At issue is only whether state legislative districts could be drawn by voting-eligible population.

The GOP's domination of state legislatures is how they have gained the power to be able to draw so many favorable congressional maps. So allowing Republicans to draw more favorable state legislative maps generally means more favorable congressional ones — and a potentially more resilient GOP majority.

Alito's question was still in the realm of the hypothetical at the time, though. That is because it has been impossible to use that voting-eligible method, given there has been no accurate census block data that included citizenship. Which is where the new Justice Department push comes in. If the question is added to the Census — which is still undecided — it could supply that data and open the door for states to actually draw districts according to who is and is not eligible to vote.

That could have a major impact in certain areas. That is because the current method has resulted in some Democratic districts with far fewer eligible voters than Republican ones. As I wrote back in 2014, Pew data showed one heavily Hispanic congressional district in California included a sizable majority (57 percent) of residents who were not eligible to vote.

That was in large part because the district was so young, but also partly because of a significant population of undocumented immigrants — as much as 15 percent of the district, according to a 2006 study from the immigrant rights-focused Immigration Policy Center. That same study showed another district in Arizona was comprised of about one-quarter undocumented immigrants.

Here is how widely congressional districts across the country varied by percentage of voter-eligible population (these are not the districts that could be redrawn, mind you, but it gives a sense for how state legislative districts also vary):

Arizona is a good example of the kind of state where this change could matter. According to that Pew data, one congressional district in Arizona has just 379,000 eligible voters — Rep. Ruben Gallego's (D-Ariz.) 7th district — while another has 566,000 — the vacant 8th, where Rep. Trent Franks (R-Ariz.) recently resigned.

Where this could matter the most, though, is Texas — the state whose governor, Greg Abbott (R), was the “Abbott” in Evenwel v. Abbott. It has the second-most congressional districts of any state, which means there is plenty of room for manipulation. In addition, its congressional districts range from as few as 335,000 eligible voters (the 33rd district in the Dallas-Fort Worth area) to 572,000 (the 21st between Austin and San Antonio).

Those districts, again, are not the ones that would be redrawn using the new standard. The state legislative districts within them could be, and that could mean a significant reddening of the state legislative map — which in turn could help Republicans stave off Democratic control and keep redrawing all the maps.

As Scherer notes, demographer Andrew Beveridge estimated in 2016 that this would help Republicans pick up several state legislative seats in states like Texas, California, Florida and New York. Redistricting is inherently a game of inches, where small shifts can determine who controls the drawing of maps that will last for a decade. For a Republican Party that has already dominated that game in recent years, a few more inches would be pretty significant.

Update: Redistricting whiz Justin Levitt makes the very valid point that, in one way, including citizenship on the Census could actually hurt the GOP. He notes that if non-citizens decline to respond, it could lead to under-counts in some GOP-controlled states that are in line to gain seats in the new Census.