My hunch is that 'liberals and leftists' live in cities and are afraid of guns and/or worry that standing up for something like the 2nd tars them with the "gun lover" moniker; they don't seem to have a problem with other issues in this way (eg, standing up for Choice doesn't make you a "baby killer" and standing up for same-sex marriages doesn't make you gay). I'm not sure you meant your question seriously, but I think it's a startling gap in awareness.
'Liberals and leftists' that I know who support the 2nd largely keep their mouths shut about it at cocktail parties.
/jordan