This Post is not about the need to compromise and work together to accomplish common goals. Although I believe that there is such a need in most cases, I can make an opposing argument. More significantly, I believe ideological purity has a more fundamental weakness.
An "ideology" is basically a collection of ideas (created by a person or persons) with some unifying theme or themes. The word is most often used in the context of political ideas. Individuals and groups have political ideologies as an attempt to explain how human interactions have operated, do operate and either will and/or should operate in order to maximize the chances of reaching certain goals the individual or group deems desirable.
No problems with that . Ideas are good. Unifying themes are necessary in order to make sense of the ideas and to explain how the world works. Unfortunately, the "unifying themes" often become "gods". As such, they become objects of faith rather than reason, and demand sacrifices in the form of ignoring evidence which might contradict the belief system. [This Post is about politics, not religion, although I will probably write something about religion in this context in the future].
Some examples of ideologies are: communism, nazism, fascism, socialism, capitalism (primarily a system rather than an ideology, but it has certainly become the latter, and, in some ways, an object of "worship"), Marxism, liberalism, conservatism, libertarianism, and anarchism.
It must be stressed that these are all systems of ideas, created by human beings, designed in large measure to describe both how the world works and how we should act politically to achieve goals, whatever they are. They are not goals (or bogeymen) in and of themselves. They are labels, describing a set of beliefs. We all like some of these ideologies and despise others, but our feelings are tied to what we believe the adoption of these ideologies have led to or will lead to if followed.
For me, the easiest example of an ideology as a "god" is Communism. Communism is based on Marx's ideas about the inevitability of certain historical forces and results. Freedom was defined as working in concert with those inevitable forces to help them progress more quickly to a higher and more desirable state. This led to a way of thinking along the lines of : "We must do X because it is inevitable that it will work". Of course, sometimes it did not. Generally, the reaction of the Communists (at least those in charge of the Soviet Union), was not to say that there was a flaw in the idea, but to either 1)claim that it failed only due to some treachery, or 2)claim that it worked. New evidence that would cast doubt on the truth of the basic ideology (or on the wisdom of the Government) was essentially ignored.
Maintaining the "purity" of the ideology and of the group of believers also becomes an end in itself. That may make religious sense, but does not make political sense. Politics is supposed to be a means to an end or ends. I could babble on about this for some time, but I will restrain myself. On this subject, I recommend reading "Homage to Catalonia", by George Orwell. In it, Orwell (one of my major heroes) describes fighting in the Spanish Civil War, on the Republican side, but after having enlisted through the "wrong" socialist group. Those running the war were more concerned with purging their own ranks than with beating Franco. Maybe Franco would have won anyway, but focusing on ideological purity certainly did not help the Republicans. The experience was central in turning Orwell from a Communist sypathizer to its implacable foe.
The main weakness of all ideologies is that they are sets of ideas with a unifying theme, while the actual world is more complicated than any unified set of ideas we mortals can come up with, particularly because it changes.
When I was a grad student in History, our professors stressed the idea that "history" was not the same thing as "the past". The past actually happened. It included the thoughts and actions of millions of people. History is merely what people write (or otherwise communicate) about the past. Even what a historian would view as a "primary" source had biases (conscious or unconscious) about what he or she reported, and also almost always had limited knowledge. Even if we had a time machine, it would only expose us to a small slice of the past. A simpler way of putting this is that "the map is not the territory".
Politics is based partly on predictions: If we do X, Y will result. Maybe; maybe not. However, when we take a set of ideas, which we tend to "freeze", as to how the world works, we will, like the Communists, tend to try to cram new evidence into the "box" of our beloved ideological structure, whether the evidence really fits into the box itself. We become more attached to the "box" - the ideology - than we are to our own powers of objective observation and analysis.
How many times have you read the phrase "The economists would not have predicted..(whatever occurred)"? Lots, I bet. The world is a big, complicted and changing place. New evidence comes in all the time. Our political theorists are not Gods; their predictive powers are limited by evidence currently available (which is often messy and fuzzy). However, assuming God does exist, He or She gave us brains and powers of observation. Ideologies are a good starting point, but they are no excuse for intellectual laziness or dishonesty. Demanding ideological purity means following false gods, or, rather, their false prophets, whose power may even be based on successfully "selling" the ideology. Enough.