California is leading a nearly 20-state lawsuit challenging the Trump administration’s abandonment of a long-standing rule governing how long migrant children can be detained.

State Attorney General Xavier Becerra and Gov. Gavin Newsom (D) announced the suit at a press conference on Monday, marking nearly the 60th time California has taken legal action against the Trump administration.

“We are ready to fight for the most vulnerable among us,” Becerra said.

Joined by Massachusetts Attorney General Maura Healey and the attorneys general of 17 other states and the District of Columbia, the lawsuit challenges new rules set forth by the Department of Homeland Security last week that effectively undo the Flores settlement.

The settlement agreement was established in 1997 after a 1985 class-action lawsuit involving several migrant children. Over the last 20 years, it has limited the amount of time the government was allowed to detain an immigrant child to 20 days or less. It also established minimum guidelines for safe and sanitary conditions in detention facilities, requiring the government to provide children with basic needs like food, water and medical care.