The Affordable Care Act is not settled law because the public remains deeply divided over it: More than half of Americans are opposed. But even more critically, congressional Republicans have withheld their stamp of approval. Many Republican lawmakers refuse even to call it a law; they keep referring to it as a “bill.”

Republicans offer several explanations for their rejection of the act’s validity. Most often, they note that the law was passed entirely with Democratic votes. This is in contrast to other major legislation, including the Civil Rights Act of 1964, which was enacted with overwhelming bipartisan support and thus became settled much more quickly.

Republicans also cite the unusual procedures used to pass the health-care act — most notably, the budget reconciliation process that avoided a filibuster while moving the final legislation through the Senate. This tactic left many Senate Republicans feeling cheated.