A federal judge in Virginia ruled on Monday that the keystone provision in the Obama health care law is unconstitutional, becoming the first judge to invalidate any part of the sprawling act and ensuring that appellate courts will receive contradictory opinions from below.

The judge, Henry E. Hudson of Federal District Court in Richmond, said the law’s requirement that most Americans obtain insurance exceeded the regulatory authority granted to Congress under the Commerce Clause.

Judge Hudson, who was appointed by President George W. Bush, declined the plaintiff’s request to suspend the act’s implementation pending appeal, meaning there should be no immediate effect on its rollout.

But the ruling seemed likely to create confusion among the public and to further destabilize political support for a law that is under fierce attack from Republicans in Congress and in many statehouses. Party leaders, including the incoming House speaker, Representative John A. Boehner of Ohio, quickly used the opinion to reiterate their call for repealing the law.