Harpers Ferry created substantial heat in the political environment. William Henry Seward of New York had been primary spokesman for Republicans in the U.S. Senate and the favorite for the party's presidential nomination in 1860. However, Seward was also perceived as an extremist, in part due to his position on the "Irrepressible Conflict" between North and South. Here it is --

http://www.nyhistory.com/central/conflict.htm

As a moderate alternative to Seward, Abraham Lincoln received the nomination. Had Harpers Ferry NOT taken place, it's likely that Seward would have been nominated and elected President in 1860. Note Seward's opposition to holding Fort Sumter and creating a conflict over a "a useless symbol of federal authority and sovereignty" --
http://www.spartacus.schoolnet.co.uk/USASseward.htm

Harpers Ferry changed the course of history in many significant ways.