Generally speaking, as is well known, the federal Supreme Court holds itself bound by the decisions of the state courts as to the constitutionality of state laws as determined by their respective state constitutions. This rule is, however, departed from in those cases in which it is conceived that it is necessary to do so in order to prevent the impairment of the obligation of contracts.

This refusal of the federal Supreme Court to follow the judgment of state courts takes the form: First, where the federal court refuses to hold itself bound by the opinion of the state tribunal as to the constitutionality of state laws which support or constitute essential elements of the contracts which, it is alleged, have been impaired by later legislation; and, Second, where the federal tribunal refuses to follow the decisions of state courts as to the constitutionality of state laws which in themselves constitute contracts upon the part of the States enacting them, and which contracts, it is alleged, have been impaired by subsequent enactments.

56 In McCullough v. Virginia (172 U. S. 102: 10 Sup. Ct. Rep. 134: 43 L. ed. 382). it is declared that "the doctrine thus announced has been uniformly followed." City Bridge Proprietors v. Hoboken Land and Improvement Co., 1 Wall. 116; 17 L. ed. 571; Wright v. Nagle. 101 U. S. 791: 25 L. ed. 921; McGahey v. Virginia, 135 U. S. 662; 10 Sup. Ct. Rep. 972; 34 L. ed. 304.