State Attorneys General Xavier Becerra (D-Calif.), Eric Schneiderman (D-N.Y.) and Maura Healey (D-Mass.), joined by 16 others sent a blistering letter Monday warning Commerce Secretary Wilbur Ross against approving a question about citizenship status in the 2020 Census.

Doing so, they said, would be unconstitutional.

Also, adding a citizenship question this late in the planning for the 2020 Census, they argued, “would significantly depress participation, causing a population undercount that would disproportionately harm states and cities with large immigrant communities.”

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“This undercount would frustrate the Census Bureau’s obligation under the Constitution to determine ‘the whole number of persons in each state,’ threaten our states’ fair representation in Congress, dilute our states’ role in the Electoral College, and deprive our states of their fair share of hundreds of billions of dollars in federal funds that are allocated in part on decennial Census data,” they wrote.

Attorneys in the Commerce Department, which oversees the Census Bureau, are reviewing a request from the Department of Justice to add the citizenship question.

DOJ said it needs the citizenship data to better enforce the Voting Rights Act.

The agency only has about a month to make its decision. The Census Bureau is by law required to provide Congress with the final wording of the census questionnaire by March 31.

“The Census Bureau has both constitutional and statutory obligations to conduct an ‘actual enumeration,’ ” the state attorneys general wrote.

“Including a question on the 2020 Census that would manipulate the count by scaring people away from being counted – causing grave harm to the states and our residents – is inconsistent with those obligations.”

Becerra said the California Justice Department is "prepared to take all necessary and legal action to protect a full and accurate Census."

“This is clearly an attempt to bully and discourage our immigrant communities from participating in the 2020 Census count,” he said in a statement.

“We also call on Congress to fully and immediately fund preparations for the 2020 Census. California simply has too much to lose for us to allow the Trump Administration to botch this important decennial obligation.”

The letter was also signed by the attorneys general from Connecticut, Delaware, the District of Columbia, Hawaii, Illinois, Iowa, Maine, Maryland, Mississippi, New Jersey, New Mexico, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington, as well as the governor of Colorado.

All those signing the letter are Democrats or independents.

This story was updated at 2:35 p.m.