Immigration activists rally outside the Supreme Court on April 23 as the justices hear arguments over the Trump administration’s plan to ask about citizenship on the 2020 Census. (J. Scott Applewhite/AP)

THERE WAS already little doubt that the Trump administration acted in bad faith when it moved to add a citizenship question to the 2020 Census. For those still unconvinced — including Supreme Court justices resistant to examining the administration’s nakedly partisan motivations — the revelation of new evidence Thursday made such willful blindness even less tenable. The court cannot let the Trump administration get away with manipulating the census in a barely disguised partisan power play.

Opponents of the citizenship question provided in federal court on Thursday documents elaborating on the partisan inspirations for the change. According to the New York Times, the daughter of Thomas B. Hofeller, a recently deceased Republican gerrymandering expert, discovered files on her late father’s hard drives suggesting that GOP operatives hoped to do more than simply depress the census count. They also wanted to give themselves an edge in legislative line-drawing.

Mr. Hofeller had concluded in 2015 that switching the way states distribute political representation — basing it on where eligible voters are, rather than on total population, whether voting-eligible or not — would benefit Republicans. Many Hispanics and their children would be excluded from consideration as legislative lines were drawn. Democratic districts would have to expand in order to encompass the same number of people. The result would be districts that skewed less Democratic. But Mr. Hofeller concluded that Republicans would need better data on the voting-age population — from, say, the census — in order to make the switch.

The documents reveal that Republicans could gain doubly from the Trump administration’s concerted campaign to add a citizenship question to next year’s census. First, the question would undoubtedly depress the count among immigrants fearful that the government would hassle them if they revealed their noncitizen status. That in itself would deprive Democratic-leaning areas of representation in the next redistricting cycle.

The data could also enable states to move away from considering non-voters at all in their line-drawing in the way Mr. Hofeller envisioned. It is little wonder, then, that Mr. Hofeller’s daughter revealed documents suggesting that he helped cook up the administration’s flimsy excuse for adding the citizenship question — to better enforce the Voting Rights Act, as though that were a priority for the Trump Justice Department.

It is unclear whether the Supreme Court would consider evidence submitted in federal district court at this late stage in its pending ruling on the move to add the citizenship question. Ideally, it should not matter much; the justices were already offered a crystal-clear picture of an administration searching for excuses to meddle with the census for partisan ends, over the concerted objections of government experts about what the shift would do to the accuracy of the count.

But, at this point, anyone paying attention would have to be purposely obtuse to conclude that the Trump administration acted for legitimate purposes. The court must not let this pass.