WASHINGTON—American voters this month turned out in historic numbers for a midterm election, but there were tens of millions who opted not to cast their ballots, and new data casts some light on who they are.

Nonvoters are far more likely to be younger, less educated, less wealthy, less partisan and black or Hispanic than the country’s voters as a whole, according to an analysis of more than 23,000 interviews of nonvoters conducted on and before Election Day by AP VoteCast, a poll conducted by the NORC at the University of...