In three nationally representative surveys of U.S. residents (N = 10 million) from 1970 to 2015, more Americans in the early 2010s (vs. previous decades) identified as Independent, including when age effects were controlled. More in the early 2010s (vs. previous decades) expressed polarized political views, including stronger political party affiliation or more extreme ideological self-categorization (liberal vs. conservative) with fewer identifying as moderate. The correlation between party affiliation and ideological views grew stronger over time. The overall trend since the 1970s was toward more Americans identifying as Republican or conservative. Older adults were more likely to identify as conservative and Republican. More Millennials (born 1980-1994) identify as conservative than either GenXers or Boomers did at the same age, and fewer are Democrats compared with Boomers. These trends are discussed in the context of social identification processes and their implications for the political dynamics in the United States.