Final answer:
Two periods marked by a general decrease in voter turnout are after the election of 1896 and since 1964, with factors including single-party dominance and restrictive laws contributing to the decline.
Step-by-step explanation:
Two time periods of considerable length characterized by a steady, general decrease in voter turnout are after the election of 1896 and since 1964. Following the 1896 election, voter participation in presidential elections dropped from about 80 percent to around 60 percent in the North by the 1920s, and to about 20 percent in the South.
This was attributed to factors such as single-party dominance, restrictive laws, and the decline of political machines. The second period beginning in 1964 saw a general downward trend in turnout rates among all age groups except for those aged 65 and over, as indicated by voting data.