Since the early days of his campaign, from his proposal to build a wall along the Mexican border to his discredited committee on voter fraud, President Trump has declared war on America’s changing demography. His administration has followed through on that strategy with a proposal to add a question to the 2020 census asking about citizenship. If the question remains on the form, millions of households, particularly Hispanic and Asian-American, could skip the census, leading to an overrepresentation of white Americans during this once-a-decade count.

Six lawsuits seeking to remove the proposed question are moving through the federal courts, with the first trial likely to take place this fall.

If it is added to the census form, the citizenship question will distort our understanding of who resides in the country. What this selective underenumeration will not do is make America’s growing racial minority populations disappear. The losers from this undercount include members of Mr. Trump’s older white base, who will suffer from lost investments in a younger generation, whose successes and contributions to the economy will be necessary to keep America great.

The demographic trends make this plain. America’s white population is growing tepidly because of substantial declines among younger whites. Since 2000, the white population under the age of 18 has shrunk by seven million, and declines are projected among white 20-somethings and 30-somethings over the next two decades and beyond. This is a result of both low fertility rates among young whites and modest white immigration — a trend that is not likely to change despite Mr. Trump’s wish for more immigrants from Norway.