Supplemental Jurisdiction

There's a lot to discuss about supplemental jurisdiction but it all begins with a determination as to whether the district court has original jurisdiction. In such instances (with some exception) the district court will likewise have jurisdiction over all claims that are so related to the claims already being heard by the district court that they form part of the same case or controversy under Article III of the U.S. Constitution. The way this is often stated is that the claim must arise from the same "common nucleus of operative fact" as a claim that properly invoked jurisdiction in the federal court.

There's an aspect of supplemental jurisdiction that comes up quite often on the MBE.  If the jurisdiction in federal court is based entirely on diversity of citizenship (as opposed to federal question jurisdiction) then the district courts will not have supplemental jurisdiction over claims by plaintiffs against persons made parties to the lawsuit under rules 14, 19, 20, or 24 or over claims by persons proposed to be joined as plaintiffs under rule 19. Likewise, supplemental jurisdiction will not apply as against those seeking to intervene as plaintiffs under rule 24. 

To clarify here, what this means is that although you should consider supplemental jurisdiction under the common nucleus test stated above, you should not consider it when a case is in federal court based solely on diversity of citizenship and when the claim offered as supplemental jurisdiction is by a plaintiff against impleaded parties (Rule 14), compulsory or permissively joined parties (Rules 19 & 20) or intervening parties parties (Rule 24). You also should not consider it when offered by persons who are to be compulsively joined as plaintiffs and by persons seeking to intervene as plaintiffs. (Rule 19 & 24)

Assume for now that supplemental jurisdiction is proper. Even so, the district courts might decline to exercise supplemental jurisdiction. This will be at the discretion of the court and the court might choose to decline for any of the following reasons: the claim raises a novel or complex issue of state law; the claim offered as supplemental jurisdiction substantially predominates over the claim or claims over which the district court has original jurisdiction; the district court has dismissed all claims over which it has original jurisdiction; or if there are other compelling reasons for declining jurisdiction.