Because they have no where to go, and will hold their noses, lean forward, and give the lever a pull (or more likely, push a button).
So I discount the ultimate importance of this report that Obama is losing his base:
With the 2012 presidential election just over 465 days away, President Obama appears to have lost crucial support from his core base of progressive voters, according to a report in the Los Angeles Times on Tuesday.
The Obama presidency has largely been defined by stubborn unemployment, a struggling economy, and a debt crisis that has taken the federal government to the edge of default. Senator Bernie Sanders (D-VT), a lead voice on progressive matters, has openly suggested the Democratic Party should field other candidates for the 2012 presidential election to compete against President Obama in the primaries.
Not going to happen.
The Democrats — particularly the left-wing “base” — never will walk out on Obama.
Like the mime in the box, deep down they believe that they have no place to go.