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"Although President Trump avers that he is bringing this lawsuit in his personal capacity, he is the sitting president and his named adversaries are entities whose decisions are subject to his direction," the judge [Kathleen Williams] wrote in an order on Friday. "Accordingly, it is unclear to this court whether the parties are sufficiently adverse to each other."

Judge Williams ordered the government and Mr. Trump's personal lawyers to submit briefs on the question, essentially forcing the Justice Department to state its position on Mr. Trump's suit. As the judge explained in her order, the Constitution requires that the two parties in a lawsuit are genuinely opposed to each other " and not colluding to engineer a legal ruling favorable to both sides. Without a conflict, the lawsuit is void and the judge must dismiss it.

"There's a requirement of adverseness," said James E. Pfander, a law professor at Northwestern University. "If the opponents are, in fact, obligated to follow the president's assessment of the law, and if the president says, It's this way and it's got to be this way,' there can be no space for a dispute."