I'm not sure if the polling is inaccurate or if the liberal blogosphere is way off the mark, but according to Gallup and this analysis by Mark Blumenthal, there isn't any statistical evidence to back up the notion that liberals are abandoning the president.
In fact, the Gallup daily tracking poll shows that the president's numbers among self-identified "liberal Democrats" have improved by three points since mid-December.
One factor in the disconnect might be that volume is being mistaken for size. I'm not sure the loudness of the kill-billers is matched by their influence. Yet.