National general election voting-eligible population turnout rates for presidential and midterm elections are plotted below, along with the raw data provided in an accompanying spreadsheet.
These numbers are taken from Vital Statistics of American Politics (CQ Press, Stanley and Niemi, eds.). Turnout rates from 1948-present are reported here and pre-1948 turnout rates are from Walter Dean Burnham, to whom I and many others are deeply indebted.
Of course, historical turnout rates are calculated from data of dubious accuracy and are at times incomplete when statistics on a class of disfranchised population -- such white male property owners who meet a religious test -- must be estimated. However, these historical turnout rates are regarded as the most accurate available.
Link to spreadsheet.