Liberalism and the Democrats have suffered a similar degradation, and for many of the same reasons. Liberal Democrats supported the Vietnam war, the defining political event of my life. Those of us who were against the war could no longer consider ourselves "liberals" so we adopted "progressive", meaning basically to the left of liberal, and antiwar. I think now "progressive" has become somewhat meaningless, since even Hillary Clinton claims to be one. In the 1960s we also called ourselves the "New Left", which is more apt, (although now we're the old left).  So is "socialist", which Bernie Sanders has revived. 

For the decline and fall of the Democratic party, see Thomas Frank's recent book "Listen Liberal". He says the Democrats have abandoned their former working-class base in favor of upper-middle class professionals. Like the Republicans, they have nothing to offer the working-class.