In his first appearance before a House committee controlled by Democrats, Commerce Secretary Wilbur Ross faced pointed questions on Thursday about his sworn testimony on his decision last year to add a question on citizenship to the 2020 census.

Democrats at the hearing, held by the House Committee on Oversight and Government Reform, expressed incredulity at Mr. Ross’s insistence that he had sought the question at the behest of the Justice Department, an explanation that two federal judges have said was not credible.

Representative William Clay, a Missouri Democrat, said one explanation of Mr. Ross’s deliberations “does not pass the laugh test,” and disclosed that a draft of the Justice Department’s request had been written by a former member of the Trump administration’s political transition team.

The committee’s questioning came ahead of a Supreme Court hearing next month on the legality of the citizenship question, which has been challenged by attorneys general for 17 states, the District of Columbia and a host of cities and counties. Critics say that adding the question will discourage immigrants from filling out the form and will improve Republican political fortunes.