Pressed on whether partisan politics colored consideration of the question, Mr. Ross said in sworn testimony to Congress in March that he was responding “solely” to a Justice Department request for data to enforce the 1965 Voting Rights Act. He also said he knew of no talks with the White House about the matter.

But that story has since unraveled.

Internal government documents produced in the principal lawsuit on the issue, in New York, show Mr. Ross pressured the Justice Department to request the citizenship question, not the other way around. They also show the involvement of President Trump’s chief strategist at the time, Stephen K. Bannon, in the discussions. After Mr. Bannon requested that Mr. Ross “talk to someone about the census,” Mr. Ross met with Kris Kobach, a fierce immigration opponent whom President Trump had appointed to a panel on voter fraud.

The federal judge in the main lawsuit ordered Mr. Ross to testify under oath, stating that “his intent and credibility are directly at issue,” but last month the Supreme Court at least temporarily blocked the testimony. Another deposition of a senior Justice Department official who worked with Mr. Ross on the question went ahead as scheduled.

The lawsuit, which goes to trial on Monday, could have profound ramifications. Census figures determine not only where hundreds of billions of dollars in federal funds are spent, but how the House of Representatives — and by extension, the Electoral College — and other political districts are remapped every decade to reflect population changes. Because immigrants and minorities disproportionately vote Democratic, a depressed head count could also expand Republican Party control when new political boundaries are drawn in 2021.

While Mr. Ross has insisted that there is no clear evidence that the citizenship question would deter people from filling out census forms, the Census Bureau’s own researchers repeatedly have found just that. In a memo last fall and in summaries of focus groups this spring, they noted that a wide range of ethnic minorities had voiced fears that the government would use citizenship information to persecute or even deport them — and one group with Vietnamese heritage even abandoned a focus group after learning that the next census would ask about citizenship.