Most consequential presidential election?

[Mr. Smith]

Wait do you mean you want this topic narrowed down to the 20th century?

Because 1860 is the obvious choice with the topic as is (with 1788 and 1796 being distant, but significant second and third places)

But making the 1700 and 1800s verboten opens up 5 big ones

Those being: 1916, 1932, 1964, 1968, and 1980. Narrowly I give it to 1932.

But 1860 easily in the grand scheme of things...easily!