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The Trump administration is adding a citizenship question to the census. Here’s why that’s bad for Democrats.

The Justice Department's request to add a citizenship question on the 2020 U.S. Census was granted. Here's how that could affect voting districts. (Video: Joyce Koh, Daron Taylor/The Washington Post)

This post has been updated with the Commerce Department's decision.

The once-per-decade U.S. Census will ask about people's citizenship status in 2020, the Commerce Department announced late Monday. The move, which came at the request of the Justice Department, reinstates a controversial question that hadn't been used on all surveys for decades.

“After a thorough review of the legal, program, and policy considerations, as well as numerous discussions with the Census Bureau leadership and interested stakeholders,” Commerce Secretary Wilbur Ross wrote, “I have determined that reinstatement of a citizenship question on the 2020 decennial census is necessary to provide complete and accurate data in response to the DOJ request.”

Although this may 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. And some are crying foul.

But why is this a big deal? Because 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. And this could significantly increase their advantages 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.
  2. Even without that, it would hand Republicans a new tool in redrawing districts even more in their favor.

The prospect of undocumented immigrants not filling out forms is the most obvious potential downside of the decision. “This will hurt accuracy in the communities where it is already hard to convince people to fill out census forms,” said Daniel Weinberg, the assistant director of the 2010 Census. He is against the move.

But the second possibility has been percolating for a while — ever since the U.S. Supreme Court's decision to take up a case called Evenwel v. Abbott. (The Washington Post's Michael Scherer had a must-read piece back in January about the Justice Department's push to bring back the census question.)

In its 2016 ruling on the case, the court unanimously ruled that the “one person, one vote” standard did not require states to draw legislative districts according to the 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 that 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.

White House proposal to ask immigration status in Census could have chilling effect, experts say

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 that there has been no accurate census block data that included citizenship. Which is where the new Commerce Department decision comes in. With this question added, it can supply that data and open the door for states to actually draw districts according to who is and who is not eligible to vote.

That could have a major impact in certain areas. That's 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 that 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 made up 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.

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.

Demographer Andrew Beveridge estimated in 2016 that this would help Republicans pick up several state legislative seats in states such as 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 a decade. For a Republican Party that has dominated that game in recent years, a few more inches would be pretty significant.

The citizenship question was included on the census through 1950, and in 2000 it was included in the long form distributed to one in every six recipients. Since 2005, it has been included on the American Community Survey, which provides estimates between censuses, but it has not been asked in any way that could provide extensive nationwide data for redistricting purposes.

Monday's change was met with criticism.

“My biggest worry is the growing risk that public confidence in the census will drop significantly,” said Terri Ann Lowenthal, who has worked for many years  as a leading census expert in the House of Representatives. “Between evidence that the administration is manipulating the census for political gain, and fear that the administration will use the census to harm immigrants, confidence in the integrity of the count could plummet. And the census is only as good as the public’s willingness to participate.”

Vanita Gupta,  president and chief executive of the Leadership Conference on Civil and Human Rights, said in a statement that her organization would “work with our coalition, the business community, bipartisan state and local officials, and other civic leaders to overturn this ill-advised decision. Adding this question will result in a bad census — deeply flawed population data that will skew public and private sector decisions to ensure equal representation, allocate government resources, and anticipate economic growth opportunities — for the next 10 years. The stakes are too high to allow this. We urge Congress to overturn this error in judgment.”

Heather Long and Tara Bahrampour contributed to this report.