From Census.gov > Topics > Public Sector > Voting and Registration > About Voting and Registration:
“Voting and Registration data have been collected biennially in the November Current Population Survey (CPS) since 1964. The statistics presented are based on replies to survey inquiries about whether individuals were registered and/or voted in specific national elections. For the purpose of these estimates, election types are considered to be either Congressional (e.g. 2002, 2006, etc.) or Presidential (e.g. 2004, 2008, etc.).”
Source: 2019 American Community Survey (ACS), 1-Year Estimates.
Note: Table 4a above is cropped on the bottom and the margin of error columns are hidden. Click on the image to see the full table with all 50 states and the District of Columbia.
From Topics > Surveys & Programs > Decennial Census of Population and Housing > About the Decennial Census: