For the key section of Chief Justice Roberts’ majority opinion, Part V, he had only the support of the court’s four liberal justices. He wasn’t doing anything fancy, just setting out the evidence about Secretary of Commerce Wilbur Ross’s decision-making process. Mr. Ross had testified to Congress in March that he added the citizenship question “solely” because the Justice Department “initiated the request” for the purpose of enforcing the Voting Rights Act, which relies on survey data the Census Bureau collects. But his emails and other materials showed a “significant mismatch between the decision the secretary made and the rationale he provided,” Mr. Roberts wrote.

That line is an understatement, but it does the job. Chief Justice Roberts showed that Mr. Ross, determined to add the question, was in search of a rationale, and that it was Mr. Ross who contacted the Justice Department, not the other way around, in hopes of finding someone to supply it.

Adding up the evidence, the chief justice concluded, as three district court judges had before him, that Mr. Ross’s “sole stated reason” about voting rights enforcement didn’t hold up. This “we cannot ignore,” he wrote. To underscore his point, the chief justice reached back to a 1977 case to extract a great line from the appeals court judge he had clerked for, Henry J. Friendly of the Second Circuit: Judges are “not required to exhibit a naïveté from which ordinary citizens are free.”

Phew.

Last year, the chief justice wrote a very different opinion for the court’s conservative wing, allowing President Trump to impose his ban on people traveling to the United States from several majority Muslim countries. In that case, the ban’s challengers also questioned the underlying motives of government officials and the plausibility of their proffered national security rationale for the ban. To give the administration a green light, the chief justice had to ignore President Trump’s many public statements maligning Muslims. The ruling suggested that the chief justice and the other conservatives on the court were willing to exhibit naïveté — to turn a blind eye to what they didn’t want to see.

But there is a difference between the travel ban and census cases. By the time the travel ban reached the Supreme Court, it was version 3.0. The government had cleaned up the worst evidence of prejudice and chaos. In the census case, the court got it before Mr. Ross built a record to plausibly justify his actions. And in the end, Chief Justice Roberts, alone among the court’s conservatives, wouldn’t pretend otherwise. Finally, there’s a limit to what the court will tolerate from the president and his officials, where no limit seemed to exist before.