On the other hand, polls that ask about, say, health-care reform without mentioning partisan affiliation tend to show that most American citizens support fairly social-democratic positions on many issues. In other words, partisan self-identification has detached itself from actual policy preferences.

I'd caution that this can be a misleading data point. Polls on universal coverage requirements or single-payer score higher than actual implementations of universal coverage or single-payer or the advocates of either position, but actual implementations or advocates are actually and meaningfully different. They're attached to things like insurance purchase mandates (which poll terribly) or drastically change the insurance market and almost all policies on it now (which poll even worse).

Other polls can be misleading because people will voice, but not vote, on a topic. The political polls on a number of matters related to gun control overwhelmingly support certain ideas(1). But the majority of the populace doesn't care about -- and in many cases, doesn't even understand -- what they're aiming to require or prohibit. It won't actually drive votes for any specific politician, and excepting where the topic can be covered by a clear ballot initiative at a time where voter turnout, won't even drive votes for a specific regulation.

(1) the exact number is usually overstated due to polling methodology, but exists.

The again, in states that have them ballot initiatives frequently produce results to the right of what the legislature is willing to pass.