With President Barack Obama poised to sign a landmark $940 billion health care reform bill into law on Tuesday, attorneys general from 12 states are preparing to challenge the bill’s constitutionality.

The Christian Science Monitor reports that a federal mandate requiring all Americans to buy health insurance or face penalties is at the center of the state AG controversy. According to the CSM, opponents of the health care bill claim the measure stretches Congress’s constitutional powers under the commerce clause regulating interstate commerce.