WASHINGTON (Reuters) - The lawmaker expected to head the powerful House Oversight Committee in the new U.S. Congress says one of his first priorities will be investigating why President Donald Trump’s administration decided to ask about citizenship on the 2020 census questionnaire.

Democratic Representative Elijah Cummings told Reuters he had not yet prioritized all the issues his committee would examine when his party takes over leadership of the House of Representatives in January.

“There are certain things that mandate that we look at immediately. One of them is the census, because that’s right around the corner,” Cummings told Reuters on Tuesday evening outside the House.

Last month, Cummings called for an official probe into why the Commerce Department added the question, which critics have said could depress the response to the census from immigrants, who often live in Democratic-leaning areas. If undercounted, areas with high immigrant populations could lose seats in the U.S. House.

Commerce Secretary Wilbur Ross announced the citizenship question in March, billing it as a way to help enforce the Voting Rights Act. The act requires a tally of citizens of voting age to protect minorities against discrimination.

Eighteen states and 15 cities have sued the administration to have the question removed, calling it unconstitutional.

U.S. congressional elections last week gave Democrats control of the House, starting in January. Incoming Democratic committee chairs are expected to bombard the Trump administration with investigations, issuing subpoenas and holding hearings.

The oversight panel Cummings is expected to chair has wide-ranging jurisdiction over the government. He has foreshadowed a broad, aggressive agenda, including a look at Trump’s business interests as well as “day-to-day” issues like prescription drug pricing and voter suppression.

As for the census citizenship question, Cummings said the oversight panel needed to “figure out why it’s there, because to be very frank with you, we have been told some untruths” by the administration. By shining light on the origins of the question, he hoped ultimately to be rid of it, Cummings said, perhaps by aiding court battles launched against it.

In October, Cummings questioned the Commerce Department’s contention that it added the question at the request of the Justice Department, saying court documents indicated Commerce initiated the idea - in coordination with former White House adviser and conservative firebrand Steve Bannon.