Why have the two major political parties (Democrat and Republican) become more polarized in recent years?

“Why have the two major political parties (Democrat and Republican) become more polarized in recent years? Please give a current example of how the parties have become polarized.

1. Title: American Government and Politics TodayAuthor: Schmidt, Shelley, and BardesEdition: Brief 11th EditionISBN: 9780357459065 2. Title: California Politics: A PrimerAuthor: Van VechtenEdition: 6thISBN: 9781544388670
   “