James Damon asked the following excellent but frustrating question:
"Can you give an example of a case where the plaintiff's claim relies on both state law and federal law but where the claim relies so heavily upon a violation of state law and to such a small degree on violation of federal law that the federal district courts fail to take the case due to lack of subject matter jurisdiction? I understand the example about the securities fraud qualifying, I want an example of something that would likely fail to meet this standard. Right now, this 'sliding scale' notion feels too airy for me to wrap my brain around."
The Supreme Court decided a case this year (Gunn v. Minton) that might help give you an idea of when a federal issue is necessary to decide a plaintiff’s action and yet not “substantial” enough to generate “arising under” jurisdiction. Then again, it might not.
The case did not concern 28 U.S.C. § 1331, but 28 U.S.C. § 1338(a), which provides for exclusive federal jurisdiction over a case “arising under any Act of Congress relating to patents.” The question was whether a state law malpractice action against a lawyer in connection with his handling of a patent case "arose under" federal patent law. The Supreme Court held no – and made it (sort of) clear that the question of substantiality doesn’t really have to do with how important the federal issue is to the case. (After all, in the malpractice action the resolution of issues of patent law would be very important because to show causation one must show that the plaintiff would have prevailed in the patent case had the lawyer not committed malpractice.) Instead, “[t]he substantiality inquiry looks to the importance of the issue to the federal system as a whole” - whatever that means. Of course Gunn may not be helpful because if the malpractice action arose under patent law, then it HAD to be in federal court. But I think Gunn does make it clear that there are lots of factors that are going to come into play in deciding substantiality.
You can take a look if you like, but I recommend you avoid this kettle of fish entirely...until you take federal courts.