Commerce Secretary Wilbur Ross in Washington on Oct. 5. (Mary F. Calvert/Reuters)

COMMERCE SECRETARY Wilbur Ross is edging ever closer to admitting that he is trying to rig the 2020 Census to help Republicans. He provided fresh evidence in a court filing last week, in which he all but acknowledged being untruthful with Congress on the matter. Unless Mr. Ross wants to be remembered for abusing his office for partisan ends, he should reverse course immediately. If he does not, Congress must rein him in.

Mr. Ross’s latest admissions came in a multistate lawsuit about his decision to add a question to census forms asking respondents about their citizenship status. Mr. Ross told lawmakers he wanted to provide the Justice Department more information on the voting-eligible population so federal attorneys could better enforce the Voting Rights Act. That’s even though the Justice Department has used other census data for voting rights enforcement without serious problems — and even though Attorney General Jeff Sessions’s Justice Department is not particularly interested in robust voting rights enforcement.

The real motivation is clear: discouraging immigrants, legal or illegal, from returning census forms. This would tend to depress population counts in cities and states where immigrants congregate — that is, blue areas. That, in turn, would result in less federal money and fewer congressional seats being apportioned to places that tend to vote for Democrats. A wide range of experts and former Census Bureau officials have objected that adding the question would make the constitutionally mandated decennial count less accurate and more expensive.

Though Mr. Ross told Congress he added the citizenship question “solely” in response to a Justice Department request, Commerce Department documents released in July revealed that he had personally pushed to add the question starting far earlier and that his staff solicited the Justice Department’s request. Now, another document shows that Mr. Ross discussed adding the citizenship question with then-White House strategist Stephen K. Bannon, Kansas Secretary of State Kris Kobach and Mr. Sessions, all immigration extremists, in spring 2017 — about a year before he approved the new question for the 2020 Census. Once again, Mr. Ross told Congress something different, testifying in March that he was “not aware” of any discussions with anyone in the White House about adding the citizenship question. (A Commerce spokesman implausibly insists that Mr. Ross was responding to a different question than the one he was clearly asked.)

Rep. Elijah E. Cummings (Md.), the top Democrat on the Oversight Committee, has promised to investigate the citizenship question decision- making if the Democrats retake the House next month. Newly empowered Democrats could also demand that the question be struck from census forms. But Mr. Ross could save them the trouble by backtracking immediately.