Lyndon B. Johnson in 1968 comes to mind. His reelection bid collapsed by March of '68 so he withdrew and allowed the Dems to chose Hubert Humphrey. George Wallace also ran and probably siphoned off some democratic votes (the old south was solidly democratic then still blaming the republicans for Lincoln and the defeat of the confederacy.) Nixon won a electoral college landslide 301 to 191 with Wallace getting 46. The popular vote was closer 43.4% to 42.8% with Wallace at just under 14%