“This seems to confirm that the Justice Department request for the citizenship question was a pretense to achieve a political goal through the census,” she said. “The pieces of the puzzle are starting to fit together, going back to when President Trump took office.”

In their lawsuit, which is led by the New York attorney general, Barbara D. Underwood, the plaintiffs imply that enforcing the Voting Rights Act was a pretext for another goal: ensuring that the nation’s 11 million-plus undocumented immigrants are not counted for the purpose of drawing congressional and other political districts, which are required to have equal populations.

The practical impact would be to reduce the number of congressional districts, and therefore Electoral College votes, in states with large numbers of noncitizens — often, though not always, Democratic strongholds.

Mr. Ross has not named the administration officials with whom he discussed the citizenship question after taking office. But other lawsuit documents released last month show that Mr. Ross received an email in July 2017 from Kris W. Kobach, the Kansas secretary of state who has taken a strong position against illegal immigration. Mr. Kobach urged Mr. Ross to add the citizenship question to the 2020 census because undocumented immigrants “do not actually ‘reside’ in the United States” but are counted for reapportionment purposes.

Mr. Kobach noted in the email that he had recently reached out to Mr. Ross “on the direction of Steve Bannon,” who was then the White House chief strategist. Documenting the extent of outsiders’ role in the citizenship decision will be a priority when the plaintiffs’ search for new evidence begins, experts said.

“That suggests very strongly that the directive here was ultimately a directive that came from the White House,” said Thomas Wolf, counsel at the democracy program of the Brennan Center for Justice at N.Y.U. School of Law.

The census tally, which includes everyone living in the United States regardless of immigration status, is used to reapportion political boundaries every 10 years to account for population changes. But a growing movement on the far right seeks to exclude undocumented immigrants from being counted during reapportionment; Alabama’s Republican secretary of state filed a lawsuit in May seeking to do exactly that.