David Horowitz, Left Illusions: An Intellectual Odyssey (Spence Publishing, 2003), pp. 274-276:
The radical commitment is less a political than a moral choice. Leaving the faith is a traumatic experience because it involves an involuntary severing of communal ties. That is why "political correctness" is a habit of the progressive mind – it is the line of fear that holds the flock in check.
No greater caution exists for those tempted to leave the faith than the charge of "selling out." Prior to the temptation, leaving the faith is inconceivable, a sign that one is no longer a good person. Only pathological behavior – a lust for money or some other benefit – could explain to a leftist the decision to join the opposition. To the progressive mind, no decent person could ever freely make such a choice. Even in the post-communist world, the most untheoretical progressive remains in this way a vulgar marxist despite all that has historically transpired. The fact that Peter [Collier] and I actually lost opportunities for personal gain as a result of our change of heart made no impression on out former comrades, who labeled us "renegades" and accused us of selling out just the same.