California Attorney General Xavier Becerra and Massachusetts Attorney General Maura Healey, along with 18 other state attorneys general, announced a lawsuit Monday challenging the Trump administration’s new rule to hold migrant families in detention indefinitely.

“This new Trump rule callously puts at risk the safety and well-being of children. It undermines a decades-old agreement reached in court by the federal government to prevent the unlawful detention of immigrant children,” Becerra said in a statement. “We’re taking the Trump Administration to court to protect children from the irreparable harm caused by unlawful and unnecessary detention.”

The rule, unveiled by the Department of Homeland Security last week, replaces the so-called Flores Agreement that set a 20-day limit for holding children.

Under the Flores settlement agreement, the government is required to release a minor from a non-licensed facility as expeditiously as possible, which has been set at 20 days.

The latest proposal, however, would give the government new licensing authority, allowing the use of either a state license or Immigration and Customs Enforcement detention standards, according to a DHS official, meaning families can be kept longer than 20 days.

The lawsuit, which is expected to be filed in the US District Court for the Central District of California, marks the first major legal challenge to the rule that has received pushback from Democratic lawmakers and immigrant advocates.

The coalition of attorneys general argue that the rule “interferes with the states’ ability to help ensure the health, safety, and welfare of children by undermining state licensing requirements for facilities where children are held,” and that it’ll lead to the expansion of family detention centers.

The attorneys general of Connecticut, Delaware, Illinois, Maine, Maryland, Michigan, Minnesota, Nevada, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Virginia, Washington and the District of Columbia are among those joining the lawsuit.

Acting DHS Secretary Kevin McAleenan acknowledged last week that the administration’s new rule would likely face litigation.

“Obviously there will be litigation,” McAleenan told reporters. “We do expect dialogue in the courts. In fact, this rule contemplates terminating the Flores settlement agreement and actually, there’s a legal proceeding just to do that coming out of the implementation. We do expect litigation, we do hope to implement it as soon as possible.”

Becerra, along with other attorneys general, also opposed the Trump administration’s initial proposal to extend detention of migrant families.

California and 19 other states sue over rule allowing prolonged detention of migrant families