Potentially, a case could be filed in state court by one party and in federal court by the other party. In such a case,federal court is prohibited from enjoining pending state court proceedings unless expressly authorized by statute, or" where necessary in aid of its jurisdiction, or to protect or effectuate its judgments. "
II. Injunctions Against Threatened State Criminal Prosecutions
Threatened state criminal prosecutions will be enjoined only when necessary to prevent irreparable harm which is clear and imminent and where appellate remedies in the criminal case are clearly inadequate to provide relief. Such injunctions are almost invariable denied, except where a federal right of free speech or assembly or a federally protected civil right is threatened by the state criminal proceeding, and it is shown that the prosecution is in bad faith or is for the purpose of harassment. Relief by declaratory judgment will ordinarily be denied if an injunction would be denied.
III. Injunction Against State Tax Procedures
28 U. S. C. Section 1341 prohibits injunctions against the assessment,levy,or collection of state taxes"where there is a plain,speedy and efficient remedy ••• in the courts of such State."
IV. The Doctrine of Abstention
Under certain circumstances the federal courts will retain jurisdiction over a suit involving a challenge to the constitutionality of a state law but abstain from deciding the question until a decision has been made by the state courts on the meaning of the state law. A determinative interpretation of the state law may obviate the federal constitutional question.
If the challenged state statute is" flagrantly and patently violative of express constitutional prohibitions in every clause, sentence and paragraph, and in whatever manner and against whomever an effort might be made to apply it," then" the federal court need not stay its hand in the face of pending state proceedings. "