Pendent [supplemental] jurisdiction, in the sense of judicial power, exists whenever there is a claim "arising under [t]he Constitution, the Laws of the United States, and Treaties made, or which shall be made, under their Authority . . . ," U.S.Const., Art. III, § 2, and the relationship between that claim and the state claim permits the conclusion that the entire action before the court comprises but one constitutional "case." The federal claim must have substance sufficient to confer subject matter jurisdiction on the court. Levering & Garriges Co. v. Morrin, 289 U. S. 103. The state and federal claims must derive from a common nucleus of operative fact. But if, considered without regard to their federal or state character, a plaintiff's claims are such that he would ordinarily be expected to try them all in one judicial proceeding, then, assuming substantiality of the federal issues, there is power in federal courts to hear the whole.
United Mine Workers v Gibbs, 383 US 715 (1966).