The decree was meant to be temporary until it could be codified into law. However, in announcing the proposed new rule, the administration said that it planned to change elements of the agreement that the advocates who negotiated it saw as fundamental — namely, a 20-day limit on detaining the families in immigration jails, after which they must be released unless they opt out by choice.

“Today, legal loopholes significantly hinder the department’s ability to appropriately detain and promptly remove family units that have no legal basis to remain in the country,” said the secretary of homeland security, Kirstjen Nielsen. “This rule addresses one of the primary pull factors for illegal immigration and allows the federal government to enforce immigration laws as passed by Congress.”

Peter Schey, president of the Center for Human Rights and Constitutional Law, who was a leader of the legal team in the original case, said he was already preparing to challenge the government in court. He and other advocates called the announcement on Thursday a thinly veiled attempt at an end run around the courts.

“The Trump administration has been whittling away at the basic rights of women and children since they came into office,” said Michelle Brané, director of the migrant rights and justice program at the Women’s Refugee Commission. “Efforts to weaken or eliminate basic child protection standards by calling them a burden or loopholes, and eliminating their obligations for the basic care of children, is just another example of the administration’s abdication of human rights.”

“The court has already had to to step in repeatedly to uphold these basic child welfare principles,” Ms. Brané added. “It is clear that the administration is incapable of holding themselves accountable.”