It’s official: After months of empty threats, the Trump administration is moving to lay siege to progressive cities, with federal grants as its weapon, in the name of immigration enforcement.

On Tuesday night, the Department of Justice announced its first real attempt to prohibit “sanctuary cities” — cities that, in the federal government’s view, don’t do enough to help federal agents enforce immigration law — from getting federal funds.

Under the DOJ’s new criteria for the Byrne Justice Assistance Grant program, the biggest federal grant for local law enforcement, states and cities will only be able to apply if they agree to allow Immigration and Customs Enforcement officers into local jails — and if they notify ICE agents at least 48 hours before anyone ICE has expressed interest in gets released.

The Trump administration’s been saber-rattling against “sanctuary cities” during its whole six months in office (not to mention on the campaign trail). But until now, the actual steps it’s taken to change federal policy to crack down on “sanctuaries” have been relatively toothless. Tuesday’s announcement puts an end to all that. It signals that the fight to defund sanctuary cities has begun in earnest.

The new grant conditions take the sanctuary city fight out of rhetoric and into policy

The Byrne Justice Assistance Grant program hands out funds to states and cities that get used for everything from training to military-style equipment. It’s the biggest grant the Department of Justice administers to local law enforcement; in fiscal year 2016, it handed out more than $263 million in funds.

The Trump administration isn’t trying to strip funding that’s already been promised. But starting in the fall, according to the DOJ’s Tuesday announcement, applicants for new Byrne grants are going to have to certify three things:

that they comply with a federal law banning any state or local policy that prohibits municipal employees from sharing information with the feds about someone’s immigration status;

that they allow ICE agents into local jails; and

that, whenever ICE sends local jail officials a “detainer request” — a request to hold someone after they’d otherwise be released so ICE can pick them up (sent when ICE agents have reason to believe someone in a local jail is deportable) — jail officials give ICE at least 48 hours’ advance notice before letting that person go.

The first criterion is in line with the steps the Trump administration has already taken against “sanctuary cities,” which have focused on the idea that cities are violating the federal law against local bans on information-sharing.

But it’s a pretty toothless critique. Even the most outspoken sanctuary cities in America claim that they don’t prohibit anyone from sharing immigration information with the federal government.

But the second two criteria added to the Byrne JAG program, though, take turn the rhetoric against sanctuary cities into actual policy.

Many cities and states limit the circumstances under which jails can hold people just because ICE asks them to. And while the DOJ isn’t officially requiring grantees to agree to all ICE requests, it’s requiring them to do things that would lead to pretty much the same outcome.

It’s going to be extremely hard — or even impossible — for many local governments to agree to these terms

Under the new conditions, local jail officials would have two options whenever they got a detainer request on someone ICE wanted to take into custody. They could honor the detainer request, and hold the person for an extra 48 hours after they’d otherwise release him to give ICE a chance to pick them up. Or they could decline the detainer request — but to remain compliant with the new terms of the federal grant, they’d still have to tell ICE 48 hours in advance that they were going to release the person, and allow ICE agents to come into the jail to collect him immediately upon his release.

Often, jail officials don’t even know 48 hours in advance when they’re going to release someone. “Local jails generally would only know 48 hours in advance of someone’s release if the person is serving out a sentence,” says Christopher Lasch, a Denver University law professor who’s assisted cities in court battles over federal cooperation. The rest of the time — “if they post bond, or if their criminal case gets dismissed,” for example — the jail officials will typically just get an order to release the person as soon as possible.

So to comply with the new DOJ grant requirements, local jail officials could end up holding an immigrant longer than they otherwise would anyway, because they’d have to wait 48 hours after notifying ICE that a release was in the works.

The Trump administration has argued extensively that picking immigrants up in jails is safer for both ICE agents and immigrants, and warned that when they can’t get easy access to immigrants in jails they resort to picking them up at home (often causing panic about raids) or in safe places like courthouses.

But for cities and states around the country that have made a concerted effort to limit their cooperation with immigration agents, making it harder for ICE to pick people up is kind of the point — they aren’t trying to create a pipeline by which someone hauled in on a traffic violation (or who’s been booked into jail during a domestic dispute in which she was actually the victim of abuse) can get sent into deportation proceedings.

The new DOJ criteria might be a very tough pill to swallow.

There’s another reason that many cities have limited their cooperation with ICE: Many state and federal courts (most recently, the Massachusetts Supreme Court) have ruled that it’s downright unconstitutional for a jail to hold someone based solely on the ICE request.

While, again, there’s nothing in the new DOJ grant criteria explicitly requiring local law enforcement to agree to ICE requests, it’s possible that the 48 hours’ advance notification requirement would end up forcing local officials to hold people longer than they would — which could run afoul of those court rulings.

The question isn’t whether cities will sue — it’s whether the Trump administration has already violated a ruling in the lawsuit that already happened

City governments have no intention of changing their immigration policies to comply with the Trump administration’s standards. But for the most part, local police departments aren’t willing to simply give up their biggest stream of federal funding to avoid an immigration policy fight.

The question is whether they can stop the new criteria from going into effect by persuading the federal courts that they’re unconstitutional.

In general, the federal government isn’t supposed to use grants to coerce cities and states into adopting its preferred policies. And the executive branch can only do so much to change grant conditions without Congress’s involvement.

But the Trump administration has a more urgent legal question on its hands: whether Tuesday’s announcement violated an existing court order.

In February, soon after the Trump administration issued an executive order that let the DOJ deny grants to cities that it deemed “sanctuaries,” a couple of them went ahead and sued over the threat of defunding. And in April, a judge sided with the cities and told the administration it wasn’t allowed to enforce that section of the executive order — except “to enforce existing conditions of federal grants” or the existing federal information-sharing law.

At first, it looked like that court ruling had forced the administration to back down, and define “sanctuary cities” as cities that violated the information-sharing law (which didn’t appear to threaten the actual funding of any actual cities, since, again, cities claim they comply with that law). But adding new criteria to the Byrne JAG funds certainly might be seen as a violation of the judge’s restriction.

It appears that that’s the point of the new announcement, though. Sessions and the DOJ aren’t waiting for the courts to decide what they can and can’t do to defund sanctuary cities. They’ve just drawn a line in the sand — one that would force many of the nation’s biggest, bluest cities to choose between losing millions of dollars for police and letting ICE agents scoop up the very immigrants cities have promised to protect.