I'm not sure who you are referring to when you say constituency. What is your definition of that term? Mine is that all people are part of the constituency: A person elected president is responsible for leading everyone; A person elected to the House or Senate is responsible for representing everyone in their district/state, not just their party. My definition of "party" are all of the people registered as Democrats.
I think you're right that some aspects of more progressive policies are not divisive for the country: I think most people would agree that everyone should have healthcare. I think most people would agree that everyone should have access to education. I'm not sure I even look at those sorts of things as "progressive". The progressive part is how those things are structured and how they are paid for.
As I think about it further, I think there's also a possibility that a majority of Democrats do believe in progressive policies on things like this, but when selecting a candidate they opted for one someone not as progressive because they didn't think the progressive one had as much chance of getting elected. Their calculation, perhaps a bit cynical, was that the less progressive person that gets elected can do more than the very progressive candidate that loses.