I was thinking about ideology of Democrats and how it has changed. For example, we can go to the progressive movement started by the candidacy and unpopular election of Donald Trump. (By unpopular I mean he didn't get the most votes and his approval rating is consistently net negative.)
But this starts before the Trump candidacy. It has been building for about two decades. This graph shows it:
The percentage of US Democrats identifying as liberal has consistently increased. It climbed 20 percentage points from 2001 to 2017, and climbed another 3 percentage points from 2017 to 2020.
Why have Democrats been consistently identifying as more liberal over time?