I feel as though there is a problematic conflation of the terms "partisan" and "ideological" in a lot of discussion about US party politics. They are treated as essentially the same when they are in fact very different and often even oppositional.
The Democratic Party, for instance, is heavily
partisan but only weakly ideological. You can see this at work on civil
liberties and war-related issues most clearly. Whereas Democrats would
have started screaming at policies under Bush, they mostly acquiesce
(unquestioningly) when such policies are championed by Obama. The same
can be said of cuts to social insurance programs like Medicare and Social Security: an unthinkable betrayal of the middle-class if under a Republican president but a pragmatic concession to the exigencies of reality if under a Democratic president. Supporting a policy because the de facto head of your
party supports it is a sign of partisanship; it has little to do with
Partisanship has its roots in in-group loyalty; it is the preferential treatment or consideration of one's own party (or group or cause). Ideology refers to the intellectual tools by which data are translated into ideas and recommendations; ideologies are how we filter the world around us and come up with a view about how the world does and should work and about what our priorities should be.