I have a lot of methodological qualms about Pew’s political typology scheme but it does help illustrate the point that the reason self-identified Democratic voters are always being disappointed by Democratic Party politicians who don’t stand up for real Democratic values is that there’s enormous internal diversity among self-identified Democrats.
Consider these polling results breaking out the three different slices of the population that Pew identifies as loyal Democrats. Among the (white, highly educated) “solid liberals” it’s taken for granted that environmental regulation and homosexuality are awesome, whereas among the (less educated, poorer, racially diverse) “hard-pressed dems” both propositions are controversial. And the rubber really hits the road here when it comes to immigration, on which both groups have lopsided—but diametrically opposed—views.
This is a big contrast from the structure of opinion among Republican loyalists, who are pretty uniformly rightwing. A Republican politician starts with a base of conservatives, and then needs to add some less conservative people. By contrast, while some Democrats have a base of “solid liberals” many don’t. And nationwide, the solid liberals aren’t just a minority of the country, they’re a minority of the Democratic Party base vote.