Congress authorized the program but never actually provided the money for it, wrote U.S. District Court Judge Rosemary M. Collyer. The program will be allowed to continue, pending appeal. 

"Congress is the only source for such an appropriation, and no public money can be spent without one," wrote Collyer, a George W. Bush appointee.

The ruling, if it stands, could be a significant financial setback for the millions of low-income Americans who benefit from the cost-sharing subsidies, which help people pay for out-of-pocket costs like co-pays at a doctor's office. It would not be a fatal blow to the future of the president's signature domestic policy achievement, but it could push insurance costs higher. 

Republicans have praised the challenge as a needed check on the White House's authority.