The judge added, “The choice as to how to devote law enforcement resources — including whether or not to use such resources to aid in federal immigration efforts — would traditionally be one left to state and local authorities.”

As a formal matter, the case, City of Chicago v. Sessions, which upheld a preliminary injunction issued by a Federal District Court last October, presented the appeals court with a straightforward statutory question: whether the Edward Byrne Memorial Justice Assistance Grant Program permits the Justice Department to attach new eligibility rules beyond those Congress established in 2006 when it created the program, which was described by the court as the “primary provider of federal criminal justice funding to state and local governments.”

Congress granted the department no such authority, the appeals court concluded, nor can the statute be interpreted as bestowing it inherently. To quote Judge Rovner: “We are faced, then, with conditions on the receipt of critical law enforcement funds that have been imposed by the attorney general without any authority in a manner that usurps the authority of Congress — made more egregious because Congress itself has repeatedly refused to pass bills with such restrictions.”

While formally statutory, the opinion is suffused with a concern of constitutional dimension about an overreaching executive branch within a system of separated powers.

“The issue before us strikes at one of the bedrock principles of our nation,” Judge Rovner wrote, “the protection of which transcends political party affiliation and rests at the heart of our system of government — the separation of powers. The founders of our country well understood that the concentration of power threatens individual liberty and established a bulwark against such tyranny by creating a separation of powers among the branches of government. If the executive branch can determine policy, and then use the power of the purse to mandate compliance with that policy by the state and local governments, all without the authorization or even acquiescence of elected legislators, that check against tyranny is forsaken.”

And if that wasn’t strong enough medicine, her opinion includes this observation: “It falls to us, the judiciary, as the remaining branch of the government, to act as a check on such usurpation of power.”

If this case or one like it reaches the Supreme Court — similar lawsuits have been filed across the country — the justices will have to take sides. There would seem to be no middle ground. Nor is there middle ground in another front in the new civil war: the Trump administration’s decision, announced on March 26, to add a citizenship question to the 2020 census. California, New York, the N.A.A.C.P., and others promptly filed lawsuits.