More can be written, but you might like to read "The US presidential election of 1876 had the greatest voter turnout in American history, with 81.8% of the voting age population voting. What were the factors that caused such a major turnout?", with answers by /u/Georgy_K_Zhukov and /u/tinyshadow .

This is not to discourage discussion. More questions, data, and debate are welcome.