What is the state's sovereign right to protect the general welfare of the people?

Appellant invokes the state's sovereign right to protect the general welfare of the people. (El Paso v. Simmons, supra, 379 U.S. at p. 508, 85 S.Ct. at pp. 583-584, 13 L.Ed.2d at p. 454.) However, "the power of a state to modify or affect the obligation of contract is not without limit." (Id. at p. 509, 85 S.Ct. at p. 584, 13 L.Ed.2d at p. 455.) The state's power is subject to the limitation of the foregoing standards.