<P> The Supreme Court has also held that only specific, "unmistakable" acts of Congress may be held to trigger the Supremacy Clause . Montana had imposed a 30 percent tax on most sub-bituminous coal mined there . The Commonwealth Edison Company and other utility companies argued, in part, that the Montana tax "frustrated" the broad goals of the federal energy policy . However, in the case of Commonwealth Edison Co. v. Montana, 453 U.S. 609 (1981), the Supreme Court disagreed . Any appeal to claims about "national policy", the Court said, were insufficient to overturn a state law under the Supremacy Clause unless "the nature of the regulated subject matter permits no other conclusion, or that the Congress has unmistakably so ordained". </P> <P> However, in the case of California v. ARC America Corp., 490 U.S. 93 (1989), the Supreme Court held that if Congress expressedly intended to act in an area, this would trigger the enforcement of the Supremacy Clause, and hence nullify the state action . The Supreme Court further found in Crosby v. National Foreign Trade Council, 530 U.S. 363 (2000), that even when a state law is not in direct conflict with a federal law, the state law could still be found unconstitutional under the Supremacy Clause if the "state law is an obstacle to the accomplishment and execution of Congress's full purposes and objectives". Congress need not expressly assert any preemption over state laws either, because Congress may implicitly assume this preemption under the Constitution . </P>

Where is rule of law located in the constitution