And so, of course, "divide and conquer" is the counter-strategy. Cointelpro (see
http://en.wikipedia.org/wiki/Cointelpro if you are unfamiliar with this history) is a case in point. The methods used are well documented even if not well known.
There will always be some among us who are susceptible to one sort of form of bias or another. All of us are, to some degree, biased and thus susceptible to manipulation. But some, due to their life experiences, will be easily convinced that disassociating from and repudiating this or that "unsavory" part of our diverse community. They can be led to believe that making "us" more pure would make "us" better. Puritans purging our ranks of the undesirables in the name of getting rid of those who "make us look bad." There is a very long history of such witch-burnings.
Objectively, however, it just makes us smaller and weaker. We need to work together for the common good, and we need to fight in many ways, in may arenas, using many tactics, acting according to many different strategies, and reflecting many different ideologies.
"E Pluribus Unum" was our national motto until the fascist forces replaced it with "In God We Trust" in 1956. Many different groups working together for the common good. That is what defines us as a common people rather than isolated, divided and ineffective factions.