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.

As I said in December 2010, when there were similar reports:

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.