Other legal experts warned the decision could have far-reaching consequences if upheld.

“This decision would be a radical expansion of the role of unelected judges to resolve disputes that are essentially political,” said Walter Dellinger, who served as acting solicitor general in the Clinton administration. “The Supreme Court has not gone down that road before, and I doubt they will go down that road now.”

Judge Collyer, who was appointed by President George W. Bush, said her decision would “open no floodgates, as it is inherently limited by the extraordinary facts of which it was born.”

But she allowed that her ruling broke new ground, since existing precedents provided no clear path.

The usual congressional responses to executive actions said to be in conflict with laws will not suffice, she said.

The administration had suggested, for instance, that Congress could cut off funding if it was dissatisfied. Judge Collyer wrote, quoting from a House brief, that the administration was “apparently oblivious to the irony” of its position.

The ruling in U.S. House of Representatives v. Sylvia Mathews Burwell came after the Supreme Court in June upheld a key element of the law and dismissed a challenge that threatened the very underpinnings of the Affordable Care Act.

A final ruling in favor of the House in the new lawsuit could undermine the health law and drive up costs for consumers if health insurance companies lose the federal aid or if replacement aid is not found, but it is not seen as having the same capacity to make the health law unworkable.

More than half of all the people with coverage purchased through public insurance exchanges — 5.6 million people out of a total of 9.9 million — receive those cost-sharing reductions, which lower their out-of-pocket costs for doctors’ services, hospital care and prescription drugs. Without that assistance, many of them would have difficulty paying their share of medical bills.