Because literally, in terms of voting records, Democrats and Republicans are almost two separate species. Following are graphs of the 2008 environmental records of the WA state legislature. Scores are voting records rated from 0-100% by the League of Conservation Voters.




House Dems: N = 66
AVG = 89.7
STDEV = 11.2
Range = 50-100

House Repubs = 38
AVG = 28.7
STDEV = 18.3
Range = 0 - 73

Numbers total more than 98 because six legislators did not serve a full term and were replaced.
Outliers defined as over 40% for Repubs (7) and under 40% for Dems (none).

Similar results for the Senate



Senate Dems: N = 32
AVG = 87.5
STDEV = 14.5
Range = 34-100

Senate Repubs = 17
AVG = 22.5
STDEV = 11.1
Range = 7-45
The one Dem outlier (under 40%) caucuses with Repubs. There are three Repub outliers (over 40%) representing purplish King County suburban areas.

I analyzed national ratings on issues important to retirees in 2005 similarly (no graphs)

House Dems: N = 201
AVG = 94.6
STDEV = 8.0
Minimum = 58

House Repubs = 222
AVG = 5.5
STDEV = 6.2
Maximum = 33

Senate Dems: N = 43
AVG = 93.3
STDEV = 10.5
Minimum = 52

Senate Repubs = 57
AVG = 8.0
STDEV = 11.5
Maximum = 69 (Jeffords)
Next after Jeffords - 35


And how many conservative Dems and liberal Repubs are there really? Counting as a conservative D someone with less than 85% rating and as a liberal R someone with more than 15%-- 7 liberal Repubs and 5 conservative Dems

Senate ConservaDems: N = 5
AVG = 71.6
STDEV = 13.0

Senate liberal Repubs = 7
AVG = 34.4 (minus Jeffords = 28.3)
STDEV = 16.5 (minus Jeffords = 6.5)

House ConservaDems: N = 19
AVG = 75.8
STDEV = 8.8

House liberal Repubs = 16
AVG = 22.2
STDEV = 4.6

So even the gap between the most liberal Repubs and the most conservative Dems is a chasm. The fly in the ointment is, of course, that when it comes to serving people rather than corporations or the military-industrial complex, the good stuff never gets out of committee. The Repubs, joined by a significant block (and even sometimes a slight majority) of Dems, block it. We are seeing this played out in health care, financial regulation and war appropriations right now. Still, much legislation, however timid, that does get out helps real people, or at least mitigates harm to same. And that is why I'm still a partisan Dem, despite of the health care and other fuckups.