As soon as Josh Block saw President Donald Trump’s tweet pledging to ban transgender troops from serving in the U.S. military, he did some tweeting of his own—in preparation of a lawsuit to stop Trump.

Block, an attorney at the American Civil Liberties Union, asked trans servicemembers, active or reserve, to contact him. His tweet got over 16,500 retweets within 12 hours. “My inbox is overflowing with messages from trans servicemembers who are serving openly with no problem,” he told The Daily Beast.

All those contacts mean Block has a lot of potential clients if and when Trump orders the Pentagon to reverse course, kick out thousands of trans troops, and bar open trans service. “There’ll be no shortage of people who’ll be ready to fight if they’re suddenly kicked out because of Trump’s decision to tweet one morning,” Block said.

Block isn’t alone. Other pro-trans civil rights groups are already scrambling to prepare the legal groundwork for challenging Trump’s latest ban in court, where his bans restricting travel to America or aspects of it have typically gone to die. Having seen the overwhelming legal opposition prompted by Trump’s Muslim ban, civil rights groups see a template in the making.

“We will go to court if this goes beyond Twitter,” said Matt Thorn, the executive director of OutServe-Servicemembers Legal Defense Network.

And for its part, a senior White House official acknowledges weeding out trans troops already serving in the military would be a “shit storm” for Team Trump.

OutServe is already partnered with the National Center for Lesbian Rights and Lambda Legal to prepare a legal challenge, according to Thorn and the Center’s legal director, Shannon Minter. While they’re in touch with the ACLU, OutServe is doing their own work, in consultation with other pro-LGBT organizations, to fight any rollback of military transgender rights.

“I’m pretty sure It’s gonna be like the Muslim ban—this administration is going to be inundated with lawsuits,” Thorn said.

Thorn and Block already have a short list of statutory predications for a court fight: the Equal Protection Clause of the Fourteenth Amendment; the Administrative Procedure Act’s ban on “arbitrary[or] capricious” measures taken by a federal entity, which includes the military; . Title VII of the Civil Rights Act of 1964 is also in the mix, though it generally does not apply to the uniformed military.

“The military already come to the very studied conclusion there’s not a single legitimate reason to exclude transgender people from service. So it would be completely apparent including to the courts that any reversal of the policy has no legitimate basis. It would be based on unrelated political considerations or animus toward transgender people,” Minter said.

Tobias Wolff, a law professor at the University of Pennsylvania who advised the Obama administration on ending Don’t Ask Don’t Tell (DADT), said constitutional challenges to military personnel policy need to contend with hostile precedents, including a 1981 case that affirmed the military has significant leeway in gender discrimination for selective service registration.

“It doesn’t mean constitutional challenges never pass muster in a military context, but it means there’s a heavier lift for plaintiffs challenging them. Some lower courts found DADT problematic even in the face of that precedent,” Wolff said.

Indeed, the legal steps are prep work for now. The civil rights groups are quick to note that a tweet by the commander-in-chief is does not have the force of an order for the military. “We don’t tweet federal policy in this country,” Thorn said. And their first priority is to build political resistance to Trump actually ordering a trans-rights rollback put into effect.

“It will be a tragedy if we end up having to litigate this issue in court. I’m very encouraged by the overwhelmingly negative response we’ve seen today by President Trump’s impulsive tweeting on this issue, including from leading Republicans who stepped forward to say it would be a gross miscarriage of justice and a disservice to our military to exclude qualified transgender people from service,” said Minter.

But if the administration moves forward, the next step on the route to the courthouse concerns seeing what the Pentagon actually does to implement Trump’s announced trans ban. And the Pentagon on Wednesday had no answer about the fate of transgender troops currently serving.

“The Secretary of Defense is waiting on guidance from the White House,” in the form of a memo, Pentagon spokesman Lt. Col. Paul Haverstick told The Daily Beast. “We can provide more guidance to the services once we have that.”

Yet at the White House, Press Secretary Sarah Huckabee Sanders punted back across the Potomac before saying she would “call it a day” if reporters kept asking her for specifics.

White House officials who spoke with The Daily Beast on Wednesday afternoon conceded that the reason White House messaging is currently vague is because an actual sweeping policy hasn’t been crafted yet.

One senior official noted that the president’s advisers are well aware of the political “shit storm dragging trans soldiers out of the military” would generate. Though the official suggested there wasn’t an appetite in the White House for that potential headache, Trump’s unpredictability made it difficult to be certain.

“There’s a lot that is going to happen before we get to the stage of new regulations being written and people being kicked out because of them,” said Block. But if the administration runs those traps, then the ACLU “absolutely” is ready to go to court.

Confusion and outright fear have been staples of what pro-trans groups have heard from their constituents in the wake of Trump’s tweets. And from the fear of vulnerable servicemembers comes resolve and anger on the part of their advocates.

“We have a lot of trans servicemembers who are very frightened by this tweet and who are doing this country a great service by putting their lives on the line to defend this country and the constitution. This is absolutely disgusting behavior by the president,” Thorn said.

— Additional reporting by Kimberly Dozier and Asawin Suebsaeng