MIT Election Data and Science Lab
They state that "Our lab is a clearinghouse for data sets that can fuel studies on elections and how they're conducted. Find election data and research tools produced by us and others."
Best place to find an election data
Voter Registration - By County and Party
United States Census Bureau
In addition to the Voting and Registration estimates made available on this website, data users can obtain Public Use Microdata Files for elections that the U.S. Census Bureau has data for via DataFerrett, the Bureau's online data access application. The November CPS data files, and entire datasets, are accessible for free through the DataFerrett tool dating back to 1994. The CPS FTP site is another location for obtaining voting and registration data.
Data users can also obtain CPS Voting and Registration data files from non-governmental websites. The National Bureau of Economic Research website contains voting supplement datasets starting in 1994. The IPUMS-CPS website maintained by the University of Minnesota includes voting supplement datasets starting in 1976.”