I'm not a Democrat, but since this thread is in North American Politics, anyone can weigh in, so I will post my two cents.
I think the main lesson the Democratic Party and, in general, many liberals and progressives need to learn is that humans tend to be driven by material conditions more than empathy, altruism, and moral slogans about issues that don't directly affect them. This is not to say that those three things can't or don't influence many people's worldviews and decisions, but when a decisive election is perceived to be a choice between someone promising a better economy, more stability, and more jobs on the one hand and, on the other hand, another who focuses more on rights and social issues that are seen by many as not directly affecting them or their living conditions, altruism will usually not outweigh voting for the option perceived to be more favorable to one's own interests.
I view the above as a descriptive rather than normative observation. I don't see those human tendencies as ideal, but they are what they are. The notion of linear "moral progress" is a myth, and so is the notion that given the "right" set of inputs or arguments, most people will necessarily be "rational" or make decisions that align with a progressive worldview (or any other specific ideology or worldview)—and I don't think a progressive worldview is necessarily synonymous with being "rational," altruistic, empathetic, etc., to be clear; I'm just framing this in terms of how a subset of liberals and progressives may see things.