In trying to keep illegal immigrant families together while maintaining a zero-tolerance policy, President Donald Trump Wednesday put that ball squarely in the court of a federal judge appointed by Barack Obama.

Reacting to a firestorm, Trump signed an executive order ending a policy of separating parents from their children when they cross the border illegally. The president wants prosecutors to keep filing criminal charges against the parents but let them stay with their children while their cases move through the justice system.

To do that, though, Trump must persuade U.S. District Judge Dolly Gee in California to reopen a 21-year-old settlement that ended a lawsuit over child detention. The executive order directs the Department of Justice (DOJ) to do just that.

Andrew “Art” Arthur, a former assistant general counsel and acting chief of the former Immigration and Naturalization Service (INS) National Security Law Division, said the executive order presents the judge with an interesting choice. She could approve the change or reject it, he said.

“And then she’s going to be the one responsible for separating families,” said Arthur, now a senior fellow in law and policy at the Center for Immigration Studies (CIS) in Washington, D.C. “It puts the onus on the courts to modify the agreement.”

That agreement, finalized in 1997, ended a suit filed in the 1980s by the American Civil Liberties Union (ACLU) on behalf of 15-year-old Jenny Lisette Flores. She had come to the United States from El Salvador, and immigration authorities detained her with adults and subjected her to daily strip searches.

In settling the suit, President Bill Clinton’s administration agreed to — among other things — limit the detention of illegal immigrant children. U.S. Border Patrol can hold them for 72 hours and then must turn them over to the Department of Health and Human Services (HHS), which has 20 days to place them with relatives in the United States or foster care.

If Gee agrees to the changes sought by Trump, the 20-day limit would be waived so that parents and children would be held together. The executive order also instructs the Department of Defense (DOD) to make space available to house the families and orders DOJ to prioritize cases involving detained families.

Christopher Hajec, director of litigation at the Immigration Reform Law Institute (IRLI), agreed that the Flores settlement is the key.

“The order can’t go in effect as the president intends unless the Flores agreement is modified,” said Hajec, whose organization advocates tighter immigration enforcement. “The president is still kind of boxed in. He’s doing the best he can.”

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Hajec said he believes the government has a good case that the circumstances have changed since 1997. But he said it is difficult to predict how the judge might rule.

“Trump is highly motivated to work out something as quickly as possible,” he said.

But New York Gov. Andrew Cuomo, who announced that his state is suing the federal government to end family separation, said on CNN Wednesday that he does not believe the Flores settlement could be changed.

“If you could have modified the decision, you wouldn’t have had the problem in the first place,” he said.

Cuomo called the executive order “more of a press release than a legal document.”

Other Democrats also rejected Trump’s solution.

“This could lead to family internment camps, Wolf,” Rep. Eric Swalwell (D-Calif.) told CNN’s Wolf Blitzer. “And that is what we do not want to see. And the way to avoid that is to still follow the Flores decision, which is a humane decision that does not want people in camps like this indefinitely.”

Swalwell added: “Indefinitely keeping them there is no better than separating parents from their children.”

A group called Value Our Families blasted the executive order, arguing it does nothing to stop “inhumane treatment” of families.

“Trump is only covering up the mess he made because he came under political pressure. Families belong in communities, not cages,” the group said in a statement. “Trump must end the cruel mistreatment and disgrace to our country.”

Related: Trump Signs Order to Keep Illegal Immigrant Families Together

Sen. Marco Rubio (R-Fla.) called out the Left.

“The ink isn’t even dry on the new executive order ending separation policy & some Democrats already arguing that keeping families together isn’t enough,” he tweeted. “Now they want them & their parents released after unlawful entry knowing full well that high % will never appear for hearing.”

Even though Trump retreated on Wednesday afternoon, he was combative during a campaign-style rally of his supporters Wednesday night in Minnesota.

“Democrats don’t care about the impact of uncontrolled migration on your communities, your schools, your hospitals, your jobs and your safety,” Trump told the rally. “Democrats put illegal immigrants before they put American citizens … What the hell is going on?”

PoliZette senior writer Brendan Kirby can be reached at [email protected]. Follow him on Twitter.