I just happened upon this survey report from NASS on state-by-state limitations on who can be in polling places. As you can easily see by skimming the list, among the states that responded to the NASS survey, there is considerable varation in who can be in polling places. This is especially true for independent election observers, as it is clear that they are either not allowed in number of the responding states, or they are required to obtain pre-election credentials to authorize their presence on election day. Enterprising students looking for research projects might think about quantifying the information in this report, and then correlating it with other measures, especially outcome measures, to ascertain whether easy polling place access by observers has any effect on outcome variables, for example, allegations of election fraud or reported incidents.