The disqualification of the acting United States attorney for the Northern District of New York, John Sarcone, from the prosecution of that state’s attorney general, Letitia James, is another setback for the Trump administration’s effort to prosecute the president’s critics. 

The disqualification of Mr. Sarcone from the case against Ms. James — he remains in his position, for now — comes from Judge Laura Schofield, an appointee of President Obama. She also dismissed the subpoenas his office had issued to Ms. James. Mr. Sarcone was investigating whether Ms. James violated the civil rights of Mr. Trump and, separately, the National Rifle Association.

The judge found that Mr. Sarcone “was not lawfully serving as acting U.S. attorney when the subpoenas were issued.” She reckoned, though, that the government could renew its pursuit of Ms. James “at the direction of a lawfully authorized attorney.” Judge Schofield determined that  “Mr. Sarcone is not lawfully serving as acting U.S. attorney. Any of his past or future acts taken in that capacity are void or voidable as they would rest on authority Mr. Sarcone does not lawfully have.”

Judge Schofield writes that “the subpoenas are unenforceable due to a threshold defect: Mr. Sarcone was not lawfully serving as Acting U.S. Attorney when the subpoenas were issued. Mr. Sarcone’s service was and is unlawful because it bypassed the statutory requirements that govern who may exercise the powers of a U.S. Attorney.”

Judge Schofield’s 24-page ruling determined that “When the Executive branch of government skirts restraints put in place by Congress and then uses that power to subject political adversaries to criminal investigations, it acts without lawful authority.” Mr. Sarcone was serving in a temporary capacity because he had never been confirmed by the Senate, a constitutional requirement. Now, he is “disqualified from any further involvement in prosecuting or supervising the instant investigations, regardless of his title.”

The government was investigating whether the civil fraud case Ms. James brought against Mr. Trump, his two adult sons, and their family business amounted to a civil rights violation. The litigation resulted in a civil judgment of some $500 million, which was overturned on appeal as so “excessive” as to violate the Eighth Amendment. Ms. James suit against the NRA resulted in millions of dollars in fines and the replacement of its entire leadership. 

Federal law mandates that in the absence of confirmation, the Executive Branch is allowed to make a single 120-day appointment. Once that deadline expires, the federal judges in the district can either extend that term or reject the choice and place their own prosecutor in the powerful post. The judges rejected Mr. Sarcone, but Attorney General Pam Bondi then appointed him a “special attorney” and insisted his term was “indefinite.” That, concluded Judge Schofield, amounted to an unlawful “workaround.”

Mr. Sarcone’s disqualification comes on the heels of similar rulings against the interim United States Attorney for the Eastern District of Virginia, Lindsey Halligan, as well as the top prosecutor in New Jersey, Alina Habba. The disqualification of Ms. Halligan was particularly galling to the Trump administration because it was accompanied by the dismissal of the criminal cases against Ms. James for bank fraud and the former director of the FBI, James Comey, for lying to Congress.

The DOJ’s woes with interim appointments flow from the Senate practice known as “blue slips,” whereby the consent of both of a state’s senators is required for a candidate for United States attorney to proceed to a confirmation vote in the upper chamber. In  practice, this has given Democratic senators carte blanche to block the path of Mr. Trump’s prosecutorial nominees to confirmation. 

Mr. Trump has declared that “Blue Slips are making it impossible to get great Republican Judges and U.S. Attorneys approved to serve in any state where there is a single Democrat Senator.” Senate Majority Leader John Thune takes the position, though, that “There are many Republican senators, way more Republican senators who are interested in preserving” blue slips.