As Donald Trump tries to expand the presence of armed forces in Democratic-run cities, he will run into a major roadblock, according to an ex-prosecutor.

Former federal prosecutor Joyce Vance, in a Substack article dated Saturday, argued that Trump is trying to "shift" the so-called Overton Window, which she describes as "a model that describes the range of policies considered acceptable at a given time by the public and policymakers."

"For instance, the idea of deploying the National Guard, or even the military, on American streets to control the local population is something we would have considered far outside of the Window for decades," according to the analyst. "Think of what Donald Trump is doing in the District of Columbia in these terms. He’s made up a crisis—a wave of crime that doesn’t exist. The law in the District is different from how it is elsewhere because of limited home rule and a law that was drafted, at least arguably, to give the president alone the ability to declare an emergency that would permit control of local law enforcement. Trump tried it in Los Angeles, but ran into issues, like the Governor’s objection and the Posse Comitatus Act, which prevents direct law enforcement by the Guard and the military. But in the District of Columbia, Trump has asserted the ability to seize control of the Metropolitan Police for at least thirty days and longstanding DOJ interpretation of the law says Posse Comitatus doesn’t apply in D.C."

For Trump, Vance says, the idea is to go further.

"Next stop, Los Angeles, Baltimore, Oakland, New York and Chicago, all cities Trump said were 'bad, very bad,' without explanation. All cities where the law is less friendly to a Trump takeover than it is in the nation’s capital," the ex-prosecutor wrote.

That's where the pitfall comes in, according to Vance.

"A potential pitfall for Trump is that outside D.C., he’ll need to convince courts, where his moves will certainly be challenged, that his determination of an emergency or other condition necessary to allow him to interfere with state and local control is not reviewable," she wrote. "Since his first day in office, when he declared an emergency at the border, Trump has been relying on that notion, that contrary to the checks and balances the Founding Fathers set up, any decision he makes that there is a national emergency can’t be challenged in the courts."

Regarding Trump's push to define these national emergencies, the pitfall seems to be slowing Trump down, Vance wrote.

"So far, the lower federal courts seem to be skeptical," she wrote with a word of caution. "At some point, that issue will make its way to the Supreme Court. If SCOTUS lets him get away with that, our position becomes that much more precarious."

Citing a specific instance in which the courts have held Trump back, Vance wrote, "On Friday, Judge Ana Cecilia Reyes, born in Uruguay and appointed to the district court in D.C. by Joe Biden in 2023, wasted no time in scheduling a hearing after the District filed a lawsuit challenging Trump’s attempt to exceed the power granted by the home rule law in his attempt to take over the Metropolitan Police. The previous night, Attorney General Pam Bondi tried to replace the D.C. Chief with the head of the DEA."

"In the end, Attorney General Pam Bondi backed down, agreeing to let Metropolitan Police Department Chief Pamela Smith continue to run the Department’s day-to-day operations under Mayor Muriel Bowser’s orders," the attorney added.

Read the full post on Substack here.