There are NO RUN-OFFS in a General Election. The top vote-getter wins; that simple. Clinton was elected two times and never got 50% of the vote.
http://www.thegreenpapers.com/G14/GA
Tuesday 6 January 2015 - Federal General Election Runoff. Polling hours 7:00a EST (1200 UTC) to 7:00p EST (0000 UTC).