[0] http://slatestarcodex.com/2014/06/14/living-by-the-sword/
But I mean cancer mostly in the same way Scott Alexander used it - uncontrolled growth by "all means necessary", not an insult. Cancer happens when some cells eschew cooperation and start promoting their interests at the cost to everyone else. As it grows in size it grows in complexity, and given that it's made of components who tend to disregard cooperation, Scott argues that instead of self-organizing it tends to develop a cancer of its own, which slows the overall growth. Similarly, if the group gains voice by using every dirty trick in the book and then some because it's most effective, they should not be surprised that - as their movement grows in size - they end up fighting each other at some point.
I hear that phrase more on the left lately than right. Which says a lot ...