Democratic Leaders Are Spineless Because Democratic Voters Like Spinelessness

Why are Democratic leaders so spineless? Ezra Klein points to persuasive polling evidence (PDF) that the Democratic likes it that way:

A more general point I would make about this is that left-wing intellectuals and writers often write as if left-wing intellectuals and writers are "the base" of the Democratic Party, then observe that the leaders of the Democratic Party aren't very left-wing, and then write about the phenomenon of the Democratic Party leaders' ignoring their base. The truth is closer to something like the actual base of the Democratic Party is the large block of voters who reliable vote Democratic and have only mildly left-wing policy preferences.