The Supreme Court is poised to decide one of its most divisive cases since litigation around the travel ban: the challenge to the Trump administration’s attempt to add a citizenship question to the 2020 census.

Even as the justices deliberate on this case, shocking new reporting offers critical support to opponents of the administration’s position. It strongly suggests that the justification from Trump administration lawyers, in their presentation before the Supreme Court, for adding a census question on citizenship was an outright falsehood, or at the least a deliberate pretext. A recently deceased Republican strategist, whose 2015 study showed that adding a citizenship question to the census would supercharge pro-Republican gerrymandering, provided the actual rationale and wrote key language that informed a Justice Department letter claiming that the citizenship question was needed to enforce the Voting Rights Act.

We have seen this drama before. Last year, the Supreme Court faced similar questions of blatant misrepresentation from the Trump administration in the case on the travel ban. The court overlooked the sketchy details in that case and decided in the administration’s favor.

In this term’s most important matter, the court should not be taken in.

During the travel ban case, I remember sitting in the courtroom astonished when the solicitor general claimed to the justices that President Trump had “made crystal-clear” that he had “no intention of imposing the Muslim ban” — a ban that, as a candidate, Mr. Trump had repeatedly promised.