"Politics was becoming more central to Americans' identities in the 21st century than it ever was in the 20th. Would you be upset if your child married a supporter of a different party from your own? In 1960, only 5 percent of Americans said yes. In 2010, a third of Democrats and half of Republicans did. Political identity has become so central because it has come to overlap with so many other aspects of identity: race, religion, lifestyle. In 1960, I wouldn't have learned much about your politics if you told me that you hunted. Today, that hobby strongly suggests Republican loyalty. Unmarried? In 1960, that indicated little. Today, it predicts that you’re a Democrat, especially if you’re also a woman. Meanwhile, the dividing line that used to be the most crucial of them all--class--has increasingly become a division within the parties, not between them."h/t Ace of Spades
A possible explanation: Government has expanded its reach continuously so that rules about behaviour and money to support non-family choices have expanded dramatically. The family is competition for the state, especially for the right to use force.