This is easily tested by comparing against the country of origin question. As it turns out, a bit over half of LW comes from the US. Wikipedia claims that about 33% of Americans identify as Democrats (vs. 28% Republican and 38% other or independent), so we'd expect about 17.5% of LW to identify as Democratic if the base rate applied, up to 35% if every American LWer identifying as liberal or socialist also identified as Democratic.

Bearing this in mind, it seems that party members identified as such really are underrepresented here.