A coalition of 20 states led by California and Massachusetts on Friday announced a lawsuit against the Trump administration's proposed rule to amend the longstanding court order that limits the detention of migrant minors to 20 days.

It's the first major litigation against the Trump administration's plan to rescind the Flores settlement agreement, a rule that would allow the federal government to indefinitely detain asylum-seeking families, including minors, until their cases are reviewed by immigration courts.

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"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," said California Attorney General Xavier Becerra Xavier BecerraState AGs condemn HUD rule allowing shelters to serve people on basis of biological sex OVERNIGHT ENERGY: Trump casts doubt on climate change science during briefing on wildfires | Biden attacks Trump's climate record amid Western wildfires, lays out his plan | 20 states sue EPA over methane emissions standards rollback 20 states sue EPA over methane emissions standards rollback MORE in a statement.

"No child deserves to be left in conditions inappropriate and harmful for their age. We’re taking the Trump Administration to court to protect children from the irreparable harm caused by unlawful and unnecessary detention. With our partners across the country, we will fight for the most vulnerable among us," added Becerra.

Acting Homeland Security Secretary Kevin McAleenan announced the move last week but quickly admitted the rule would likely face stiff opposition in the courts.

The lawsuit, which is scheduled to be filed later Monday, argues that the Trump administration's proposed rule will endanger minors by removing state licensing requirements for family detention centers.

The states also argue that the expansion of family detention will lead to an uncontrolled expansion of for-profit detention, creating incentives for more families to be detained, and that prolonged detention under any circumstances has negative long-term medical consequences for children.

In the press conference announcing the rule, McAleenan touted the conditions at family detention centers, including health care and education standards, saying, "Furniture, bedding, towels, clothing and toiletries are provided."

The lawsuit alleges the rollout of the rule violates the Administrative Procedure Act and the due process clause of the Fifth Amendment to the Constitution.

The Flores settlement agreement was first imposed on the federal government in 1997 as a result of a lawsuit. It was amended in 2015 to make its benefits extend to migrant minors traveling as part of family units.

Monday's lawsuit is the 13th filed by California against the Trump administration on immigration matters, and the 57th filed on any issue.

In July, California sued the federal government, saying it was not providing soap, clean water or toothbrushes to migrants detained based on the regulations imposed by Flores.

"This has become a very frequent sight, but we're not going to stop doing what we need to do unless the federal government stops trying to change the law by breaking the law," said Becerra.

"The state has interest in this. California, its economy, its people, will be impacted," Becerra added. "We're going to go in as a party because the interests of our state are at stake."

Becerra also announced the state is filing a preliminary injunction against the Trump administration's public charge rule.

Under that proposal, set to take effect in October, migrants who legally use or have used Social Security or other benefits would face significant obstacles in obtaining permanent residency or citizenship in the future.

Activists have argued the public charge rule would especially hurt American citizen children of immigrants without legal status, as their parents would be disincentivized from using social programs.

"Public charge is nothing but family separation by any other name — mother has to decide whether to feed her kids," said California Gov. Gavin Newsom Gavin NewsomOVERNIGHT ENERGY: California seeks to sell only electric cars by 2035 | EPA threatens to close New York City office after Trump threats to 'anarchist' cities | House energy package sparks criticism from left and right California seeks to sell only electric cars by 2035 EPA head questions connection of climate change to natural disasters MORE (D).