President Trump’s decision not to include a question about U.S. citizenship on the 2020 Census questionnaire appears to put the issue to bed for another decade. Don’t believe it.

The fact is, the Census Bureau already collects data on the number of noncitizens living in the United States. Its annual American Community Survey includes a citizenship question on more than 2 million surveys each year. The bureau then uses that data to estimate the number of citizens and noncitizens living in each state, county and city. Indeed, the bureau’s data allows it to estimate the number of noncitizens living in each political district in the United States.

The phrase “one person, one vote” is the cardinal principle that governs the constitutionality of our entire redistricting system. As established in a trio of cases in the 1960s, that principle means that every person’s vote must, as far as is practicable, have the same weight as anyone else’s when it is cast. Aside from the U.S. Senate, which has an inherently unequal structure protected by the Constitution, this has meant that districts must be drawn to be nearly equal in total population as of the time of the census.

But these cases were determined at a time when the share of foreign-born noncitizens was at a historic low in the United States. There are between 22 million and 25 million noncitizens currently living in the United States, according to Census Bureau data. Under current law, these people will be included in data used to redraw district lines at all levels of government even though they are not legally able to vote. This simple fact, combined with the noncitizens’ residential patterns, means that some votes will count more than others — in many cases, by a lot more.

Consider California, the state with the largest share of noncitizens within its total population. As of 2017, the total population of the state’s 53 U.S. House seats only varied a little, from a high of 826,899 to a low of 705,588. But the total population of citizens of voting age varied significantly, from a high of 578,423 to a low of 346,059. This means that votes are not of equal value in California congressional races; votes by citizens living in areas with large numbers of noncitizens have greater impact because fewer votes are required in their districts to elect a representative than are required in high-citizen seats.

This vote value disparity increases the smaller the districts get. California has 80 state assembly seats, each representing a population around 480,000 people. ACS data from 2017 show that there are more than 100,000 noncitizens in seven districts, reaching a high of nearly 168,500 noncitizens in Assembly District 53, located in Los Angeles. On the flip side, there are eight districts with fewer than 35,000 noncitizens reaching a low of only 14,000 in the far northern Assembly District 1. A vote cast for a low-noncitizen seat can be worth as little as half or less the value of a vote cast for a high-noncitizen seat.

These differences were baked in when the seats were redrawn after the 2010 Census. At the time, the number of citizens of voting age differed wildly even though each seat’s total population was nearly identical. The number of voting-age citizens in San Diego’s Assembly District 78 was 352,199 while that of District 53 was only 164,102. The vote cast in District 78 had less than half the value of one cast in District 53. Under the principles governing redistricting law, this is plainly unconstitutional.

These practices have serious partisan effects. While the existence of noncitizens in the redistricting pool does not inherently favor one party over another, their residential patterns (they tend not to live near large numbers of Republican voters) means Democratic candidates are the main beneficiaries. Republicans hold only 18 of California’s 80 Assembly seats. Twelve of those are among the 17 districts with the lowest numbers of noncitizens, and two others were won by Democrats only last year. Every seat in the 17 with the highest number of noncitizens has been safely Democratic for the entire decade.

Note that I am referring only to redistricting, not to its cousin, reapportionment. Apportionment determines how many seats in the U.S. House a state is entitled to. The Constitution clearly established that this number is determined by the total number of people residing in each state. Accordingly, California will not lose any congressional seats if we switch to citizen-based districting. But Democrats in California might lose some seats.

Australia has an even larger share of foreign-born residents in its population than does the United States, but it requires its districts to be drawn to equalize the number of electors, not the total population. As in the United States, Australia apportions the number of seats each state receives in its House of Representatives according to that state’s total population, including noncitizens. It thus preserves the one-person-one-vote principle while providing that noncitizens receive indirect representation by giving the states in which they live more seats than they would otherwise receive.

Democrats will howl, but the current redistricting system clearly is inconsistent with the one-person-one-vote principle. We should expect Republican-affiliated lawyers to make this argument in court after the post-2020 redistricting, thereby placing this issue squarely before the Supreme Court mid-decade. Don’t be surprised if the court strikes down plans in states with large numbers of noncitizens and requires inclusion of a citizenship question in the 2030 Census.

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