The current partisan divide in America

What are the principal factors contribution for the current partisan divide in America?
American Politics is more polarized than any time in the last century. What are the principal factors contribution for the current partisan divide in America and what, if any, reforms to our political institutions (i.e., elections, parties, legislatures, executive, courts) do you think might help heal the partisan divide?
My stance is that it is very unlikely for us to heal our partisan divide and if it does heal, it would only be temporary. However, if you find sources on a different stance then you're welcome to change it anyway you want.