Scholars who are interested in accessing additional data on political history, U.S. demographics, and other geospatial data may find the following sources helpful. In many cases, data may not already be prepared for using in ArcGIS, but users may code it to join with boundary shapefiles.
NHGIS, or National Historical Geographic Information Systems, is a project by the Minnesota Population Center at the University of Minnesota. This is one of the first systematic efforts to provide general interest historical GIS resources. Most important of their data is U.S. Census information going back to the first census, and shapefiles for census tracts going as far back as tracts were created for cities. The state boundary shapefiles for Mapping Congress originated with NHGIS.
Vote View is an effort led by political scientists Kenneth Poole and Howard Rosenthal to document the activities of Congress and to quantify and visualize policy votes. The roll call votes used by Mapping Congress for the U.S. Congress were digitized by the Vote View project.
The Inter-University Consortium for Political and Social Research is a group of more than 700 academic institutions and research organizations, that archives data and trains researchers in data access, curation, and methods of analysis for the social science research community. Congress of the Confederacy data and U.S. Congress elections data are stored by and are provided by ICPSR.