There is a wealth of literature on 1. the decline in mass political party affiliation 2. the decline in participation in the popular vote 3. the decline in working class self-organisation
See just as an example Hay Why we hate Politics; The European Values Survey; EJ Dionne, Why Americans Hate Politics; Sennett Decline of Public Man; Putnam Bowling Alone etc etc
These are all well supported empirically.
Fewer people participate in public life, their identification with political parties is less strong than their parents' was, their voting loyalties are more erratic than they were, and the long-term trend is to vote less in elections.
That is the background to any difficulties that the Democrats may have with their popular base.