WASHINGTON — A decision by the Trump administration to add a question on citizenship to the 2020 census discriminates against ethnic minorities for political gain, violating the Constitution, argues a federal lawsuit brought Thursday on behalf of several groups representing minorities.

The lawsuit, filed by 21 organizations and two citizen plaintiffs in United States District Court in Maryland, is the fourth major effort to block a citizenship question from the 2020 tally. The first suit, filed in April by a mostly Democratic coalition of state attorneys general and cities, argued that the citizenship question violated the Constitution’s requirement that a census count everyone residing in the United States, as well as laws governing data quality and administrative procedure.

The latest suit repeats those claims, but also argues that the administration added the question to deter Asian-Americans, Latinos and some immigrants from completing census forms for fear that the information would be used against them or members of their household, and thus undercount them in the final census tally. A serious population undercount, the plaintiffs said, would reduce minority representation in the House of Representatives and state and local governments when political districts are reapportioned early next decade.

That violates the Fifth Amendment’s equal-protection clause, the lawsuit states — the first time that opponents of the citizenship question have leveled a charge of intentional discrimination against the administration for its census decision.