Eighteen states and six cities are suing to prevent the Trump administration from including a citizenship question on the 2020 Census.

The states, all led by Democratic attorneys general, and cities argued that seeking citizenship information in the upcoming Census would depress participation, especially among states with large immigrant populations.

An undercount would then hurt states’ representation in Congress and the Electoral College, and threaten billions of dollars in federal funds allocated to states for programs like Medicaid, attorneys general for the 18 states said in the lawsuit.

The lawsuit was filed in the U.S. District Court for the Southern District of New York.

“This decision will ‘inevitably jeopardize accuracy of the population count’ by significantly deterring participation in immigrant communities, because of concerns about how the federal government will use citizenship information,” the attorneys general said in the lawsuit, citing a decision from a 1980 case. “These concerns have been amplified by the anti-immigrant policies, actions, and rhetoric targeting immigrant communities from President Trump and this administration.”

The Department of Commerce announced last month it would include a citizenship question on the decennial Census. The Trump administration argued the citizenship information was needed for better enforcement of the Voting Rights Act.

“One of the federal government’s most solemn obligations is a fair and accurate count of all people in the country, citizen and non-citizen alike,” New York Attorney General Eric Schneiderman said. “For decades, administrations from both parties have treated this constitutional requirement with the respect and reverence it deserves. Now, the Trump administration is breaking with that tradition — recklessly abandoning nearly 70 years of practice by demanding to know the citizenship status of each resident counted.”

Schneiderman said immigrants are currently “living in fear,” and warned the inclusion of a citizenship question on the Census would “drive them into the shadows.”

The lawsuit was brought under the Constitution’s Enumeration Clause and the Administrative Procedure Act.

Attorneys general from New York, Connecticut, Delaware, Illinois, Iowa, Maryland, Massachusetts, Minnesota, New Jersey, New Mexico, North Carolina, Oregon, Pennsylvania, Rhode Island, Virginia, Vermont, Washington, and the District of Columbia signed onto the lawsuit.

New York City, Chicago, Philadelphia, Providence, San Francisco, and Seattle, as well as the U.S. Conference of Mayors, joined the 18 attorneys general.

The lawsuit from the 18 states is the second challenging the inclusion of a citizenship question on the 2020 Census. California Attorney General Xavier Becerra is also suing the Trump administration over the addition.