Biden

He was a long time senator, a vice president and president but he decided not to run for reelection. While he personally thought he could serve another term the polls showed he might lose. The country was deeply divided and his party thought he could not only lose the White House but would cause loses down ballot. When other congressmen saw their reelection chances being diminished the pressure built and this was followed by a drop in campaign donations and finally many elected officials publicly asking for him to drop out. His VP was closely aligned with his policies and that worried the insiders. His party saw the republican candidate as a threat to democracy and scrambled to devise a new winning strategy. This all happened in 1968 when LBJ dropped out. To quote Winston Churchill, "those who cannot remember the past are condemned to repeat it".