You can support the mainstream positions of both group A and group B, but be more critical of group B due to the existence of a subgroup you consider malign.

Yes of course, and most political (or other) groups are filled to the brim with self criticism. And this is very difficult to confuse with the phenomenon I am describing. In particular, if you only ever talk about how horrible subgroup B is and never suggest ways of improving the movement and you use the same language to describe subgroup B and the movement at large then you are probably engaged in politics and not constructive criticism.

This is a good example of in group criticism. The author calls out members of his political affiliation for making a stupid argument, presents an alternative framework for understanding the issue and moves on.

Repeatedly talking about how horrible subgroup B is not an example of in group criticism.

Meta-level point: out group criticism can be well reasoned and valuable. It is more often a politicized rhetorical weapon. The phenomenon I am describing is usually the latter.