In the American context I would add the Nationalists/Federalists beating out the so-called "Anti-federalists" to get the Constitution adopted. But then there are also many elections that could be named as well (1860, 1876, 1912, 1916, 1964, 2020)...
I was just thinking of war (despite your initial question) IMO the most calamitous US election was 1896, in which the conservative, free trade party (Democrats) was taken over and we haven't really had a proper conservative party ever since (although of course the GOP is closer to it than modern Dems)