(Reuters) - In his statement Tuesday revoking the Obama administration policy known as DACA, Attorney General Jeff Sessions invoked a 2015 ruling by the 5th U.S. Circuit Court of Appeals that struck down a parallel Obama program to protect some undocumented parents of children born in the United States from deportation, or DAPA. The appeals court determined, among other things, that the Obama administration did not follow the proper administrative procedures to implement the DAPA program. Sessions said the Trump Justice Department concluded that DACA was similarly flawed and likely to be enjoined if it were challenged in court.

Litigation to keep DACA in place is imminent. The attorneys general of New York and Washington announced plans to sue on Monday, before AG Sessions even disclosed the Trump administration’s formal policy.

The National Immigration Law Center, meanwhile, on Tuesday afternoon sought consent in Brooklyn federal district court to amend ongoing litigation over the revocation of an undocumented immigrant’s work permit to a sweeping challenge to the Trump administration’s DACA rescission.

The group argues that the new Trump policy violates the Constitution’s Equal Protection and Due Process clauses, pulling protection out from under U.S. residents who relied on the government’s assurances. According to the letter NILC and its co-counsel sent to U.S. District Judge Nicholas Garaufis, the group intends to assert the argument that the rescission of DACA “is unconstitutionally motivated by anti-Mexican and anti-Latino animus.”

But the strongest immediate legal case to block the new Trump policy, according to two legal experts, is probably going to be the same wonky argument that led to a nationwide injunction against Obama’s DAPA program: The Trump administration, according to this argument, violated the Administrative Procedure Act by rescinding the Obama policy without engaging in a formal notice-and-comment process.

Federal courts are supposed to avoid reaching constitutional issues if cases can be decided on other grounds. So at the very least, it’s likely that any federal judge considering a challenge to the Trump administration’s decision to rescind DACA would look first at arguments that the revocation was procedurally problematic.

It may seem intuitive that a policy enacted without formal rulemaking can also be rescinded by executive branch fiat, but that’s not necessarily true, according to University of Chicago law professor Daniel Hemel. DACA – which, as you know, allows undocumented young people who were brought to this country as children to apply for a formal two-year deferment of any deportation action, as well as a permit to work legally in the United States – was put in place via a 2012 memo from Obama’s Homeland Security Secretary, Janet Napolitano, after Congress failed to enact legislation to protect the so-called Dreamers. Napolitano’s memo characterized the DACA application process not as the creation of a new, substantive right for Dreamers but as a case-by-case exercise of the executive branch’s prosecutorial discretion.

That’s important, Hemel said, because prosecutorial discretion doesn’t require notice and comment under the Administrative Procedure Act. There’s at least a plausible argument that the Obama administration didn’t skip over necessary procedures when it implemented DACA.

But if, in revoking the Obama DACA policy, the Trump administration is ruling out deferred deportation for Dreamers, Hemel said, it is creating a substantive rule. And the APA does not allow executive agencies like the Justice Department and the Department of Homeland Security to make substantive rules without a formal process.

“It’s a textual argument,” said Hemel, who wrote about the APA challenge to the Trump DACA rescission at the Take Care blog. “There’s no carve-out” allowing the executive branch to abandon administrative procedures to revoke rules it believes previous administrations adopted improperly, Hemel said. (A second legal expert also told me DACA challenges are likely to turn on the APA but asked not to be named.)

Put more simply, DACA supporters will argue that the Trump administration was arbitrary and capricious in revoking the Obama policy. The government “merely made the erroneous assertion that the executive branch lacks the authority to maintain such a program, even though no court has decided (its) legality,” wrote NILC and its co-counsel in their letter Tuesday to Judge Garaufis, seeking permission to challenge the new Trump policy. “In today’s abrupt policy shift, the government has failed to provide a reasoned explanation for disregarding the findings and conclusions that underlay the 2012 DACA memo, in violation of the APA.”

In his essay at Take Care, Chicago prof Hemel cited three cases in which the District of Columbia U.S. Circuit Court of Appeals specifically said that defects in an underlying rule are not justification for revoking it without notice and comment. The most recent is 2013’s American Forest Resource Council v. Ashe, a challenge to the Fish and Wildlife Service’s revision of the rule designating protected habitats of the marbled murrelet, an endangered seabird. The D.C. Circuit said in American Forest that except in rare circumstances, such as if a court has found an executive branch rule to be defective on the merits, “an agency rule may not be repealed unless certain procedures, including public notice and comment, are followed, and that this is true even where the rule at issue may be defective.”

The Supreme Court hasn’t ruled on this particular question. Nor has it ruled substantively on the Obama administration’s circumvention of notice and comment procedures when it implemented immigration policies to defer certain deportations. The justices reviewed the 5th Circuit’s DAPA decision before Justice Antonin Scalia’s empty seat was filled. The court split evenly on overturning the appellate ruling and affirmed the 5th Circuit’s judgment without additional discussion of the case.

The Administrative Procedure Act is not, by itself, going to save DACA even if courts agree that the Trump administration must subject its rescission of the Obama policy to notice and comment, said UC Hastings law professor Zachary Price. “I don’t think it could get you back to re-instatement,” Price said. But if challengers convince a federal judge that the new policy was substantive rulemaking, Hemel said, the judge could order the administration to keep DACA in place until it satisfies procedural requirements. That would buy time for DACA recipients facing the expiration of deferrals and work permits.

In the end, as AG Sessions said in his statement Tuesday morning, it’s going to be up to Congress to protect Dreamers. Whatever the ironies of his comments, considering this administration’s position in the travel ban litigation on the wide sweep of the president’s power to set immigration policy, the attorney general could not have stated more forcefully his belief that DACA is a legally indefensible violation of constitutional separation-of-powers doctrine. Sessions is not going to change his mind. (The Justice Department did not reply to a request for comment.)

So groups hoping for a reprieve for Dreamers ought to make a priority of lobbying, which can provide a more permanent solution than litigation. But for now, the law that kept undocumented parents in the shadows could allow adults brought to the U.S. as children to remain in the light.