The political parties change their shape—their leaders and policies—in order to win political power. In their current forms, they contribute to dividing the populace and intensifying animosity.
One major reason is that they both take for granted the outcomes in more than 40 states, leaving as few as six as the contested battlegrounds that determine the electoral college outcome. In those states, the two parties focus on turning out their base and then appealing to swing voters, but those swing voters do not necessarily represent in full the views of most swing voters in the country as a whole.
As a result, the presidential election system does not encourage either party's nominee to conduct a unifying, holistic campaign. Instead, the two major parties maximize negative campaigning, reflected in the content of their advertising and the themes of policy proposals. After such divisive elections, the country remains as factionalized and internally discontent as before the voting takes place.