http://democracy.ru/english/library/international/eng_1999-11.htmlThere are four principal situations where deferral to state or local enforcement authorities may not be appropriate. If any of these factors is present in a pattern of conduct, it may be appropriate to prosecute it federally -to the extent that this is possible under available federal laws. These four situations are:
1. Federal affect. Where the objective of the conduct is to corrupt the outcome of a federal elective contest, or where the consequential effect of the corrupt conduct impacts upon the vote count for federal office (e.g., Anderson v. United States, 411 U.S. 211 (1974)).
2. Civil rights. Where the object of the scheme is to discriminate against racial, ethnic or language minority groups, the voting rights of which have been specifically protected by federal statues such as the Voting Rights Act, 42 U.S.C. §1973 et seq.
3. Prosecutor of last resort. Where federalization is required in order to redress longstanding patters of electoral fraud, either at the request of state or local authorities, or in the face of longstanding inaction by state authorities who appear to be unwilling or unable to respond under local law.
4. Link to other crimes. Where there is a factual basis to believe that fraudulent registration or voting activity is sufficiently connected to other from of criminal activity that perusing the voter fraud angle will yield evidence useful in the prosecution of other categories of federal offense.