California Attorney General Xavier Becerra declared on Monday evening that he will sue the Trump administration after Commerce Secretary Wilbur Ross announced that a citizenship question will be added to the 2020 Census.

“We’re prepared to do what we must to protect California from a deficient Census.” Becerra said in a statement. “Including a citizenship question on the 2020 census is not just a bad idea–it is illegal.”

Earlier in the day, Becerra co-authored a San Francisco Chronicle op-ed with California Secretary of State Alex Padilla in which they argued that the “truly insidious” citizenship question “would discourage noncitizens and their citizen family members from responding to the census, resulting in a less accurate population count.”

In February, Becerra, who along with 18 other attorneys general asked Ross to not include the citizenship question, put the Trump administration on “notice” and said he was prepared to “take any and all necessary legal action” if the Census included a citizenship question:

The California Department of Justice is putting President Trump on notice: if a citizenship question is added to the 2020 U.S. Census Bureau questionnaire, we are prepared to take any and all necessary 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. 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.

Ross pointed out on Monday that “between 1820 and 1950, almost every decennial census asked a question on citizenship in some form.  Today, surveys of sample populations, such as the Current Population Survey and the ACS, continue to ask a question on citizenship.”

“For the approximately 90 percent of the population who are citizens, this question is no additional imposition,” Ross added. “And for the approximately 70 percent of noli-citizens who already answer this question accurately on the ACS, the question is no additional imposition since census responses by law may only be used anonymously and for statistical purposes.”