The discussion of θεωρια and φρονησισ (roughly meaning theory and practice) in Aristotle's Nicomachean Ethics is very helpful in understanding what critics mean by 'ideological politics' and how non-ideological politics is even possible (something ideologues often doubt).
I'm working from a translation that renders 'φρονεσισ' as 'prudence,' which is fine, as long as one understands what is meant by that is practical sagacity, and not timidity or an unwillingness to take risks. Aristotle differentiates theoretical and practical knowledge: the former is about universals and gives us necessary truths, while the latter has more to do with particulars than universals and its truths are less certain:
"Scientific knowledge is supposition about universals, things that are by necessity... Prudence, by contrast, is about human concerns, about things open to deliberation... Nor is prudence about universals only. It must also acquire knowledge of particulars, since it is concerned with action and action is about particulars."
The difference between the two realms is significant enough that someone may be very sharp theoretically but a 'klutz' in terms of practical action:
"That is why in other areas also some people who lack knowledge but have experience are better in action than others who have knowledge... Indeed [to understand the difficulty and importance of experience] we might consider why a child can become accomplished in mathematics, but not in wisdom or natural science. [This surely applies far less to today's highly abstract physics than to Aristotelean natural science.] Surely it is because mathematical objects are reached through abstraction, whereas in these other cases the principles are reached from experience."
The above, in fact, points to a good definition of what constitutes a political ideology: An ideology tries, by creating an abstract world of political 'principles' (e.g., the 'non-aggression principle') to make politics, a practical activity requiring experience, into a theoretical activity that even a bright child can become adept at through textbook learning. As Oakeshott would have it, an ideology is a 'cheat sheet' for those lacking political experience.
However, the creation of an ideology rests on a confusion:
"It is apparent that prudence is not scientific knowledge; for, as we said, it concerns the last thing [i.e., the particular], since this is what is achievable in action. Hence it is opposite to understanding. For understanding is about the [first] terms, [those] that have no account of them; but prudence is about the last thing, an object of perception, not of scientific knowledge."
By mistakenly equating political prudence with scientific knowledge, the ideologue has made a crippling error. Not that he can actually conduct politics as a sort of theoretical activity: in fact, he will again and again fall back upon disguised practical reasoning in forming his supposedly theoretical conclusions. But the contortions involved in doing so are damaging; as Collingwood said, "A person may think he is a poached egg; that will not make him one: but it will affect his conduct, and for the worse."