Section 1367(a) is a broad grant of supplemental jurisdiction over other claims within the same case or controversy, as long as the action is one in which the district courts would have original jurisdiction. The last sentence of § 1367(a) makes it clear that the grant of supplemental jurisdiction extends to claims involving joinder or intervention of additional parties. The single question before us, therefore, is whether a diversity case in which the claims of some plaintiffs satisfy the amount-in-controversy requirement, but the claims of other plaintiffs do not, presents a “civil action of which the district courts have original jurisdiction.” If the answer is yes, § 1367(a) confers supplemental jurisdiction over all claims, including those that do not independently satisfy the amount-in-controversy requirement, if the claims are part of the same Article III case or controversy. If the answer is no, § 1367(a) is inapplicable and, in light of our holdings in Clark and Zahn, the district court has no statutory basis for exercising supplemental jurisdiction over the additional claims.545 U.S. at 558.
