In a foreboding ruling for the Obama administration, a federal judge in Florida decreed Thursday that a legal challenge to the new health care law by officials from 20 states could move forward and warned that he would have to be persuaded that its keystone provision  a requirement that most Americans obtain insurance  is constitutional.

“At this stage in the litigation, this is not even a close call,” wrote Judge Roger Vinson of Federal District Court in Pensacola, Fla., before asserting that the insurance mandate was an unprecedented exercise of Congressional authority.

“Of course, to say that something is ‘novel’ and ‘unprecedented’ does not necessarily mean that it is ‘unconstitutional’ and ‘improper,’ ” Judge Vinson continued. “There may be a first time for anything. But, at this stage of the case, the plaintiffs have most definitely stated a plausible claim.”

Judge Vinson, a senior judge who was nominated by President Ronald Reagan, indicated last month that he would let the case proceed. In Thursday’s opinion, he formally rejected the federal government’s motion to dismiss the suit. It now proceeds to a full hearing on the constitutional issues on Dec. 16.