A highly regarded political scientist, David Mayhew, says it was the election of 1860, followed by 1932. He outlines different factors that make for important elections.
1 - Was it considered especially important at the time?
2 - Was the election associated with major, long-lasting change in voter coalitions?
3 - What if the other guy had won?
4 - Did the campaign itself have a big impact, independent of the result?
5 - Did it set a landmark political precedent?
He mentions those of 1800, 1828, 1896, 1912, and 1968, among afew others, but surprisingly, given ongoing partisan spats, he argues that none since 1980 were especially important.
