In 1980, David Zarefsky warned that the way that rhetoricians studied social movements took for granted the agitations and discourses of a movement at their word. Namely, he said, what guaranteed that a social movement always existed outside the existing relays of power in a given political space besides its own claims to be a "social movement"? Rhetorical studies, he argued, risked deciding in advance what was and was not a social movement by assigning certain movement-specific content a transhistorical role indetermining the formal characteristics of what counted as a social movement.
Zarefsky's alternative was to always insist on historically situated studies of the rhetoric of particular controversies and demands. Through close historical analysis, one could presumably figure out at a given synchronic moment whether or not one was seeing an oppositional or hegemonic social movement. Zarefsky's form/content argument strikes me as absolutely correct: just saying you're a social movement doesn't make it so, and the left does not hold a monopoly on discourses of social agitation.
One is also tempted to fill out Zarefsky's objection through a recourse to post-structural theories of political space and identity. Namely, what is at stake in even announcing the existence of something like a "social movement" implies the existence of something like a total/fixed/settled field of the political and its constitutive discourses at a given moment. The danger, of course, is analagous to the recurrence of the "determination in the last instance" problem faced by those attempting to make critical gestures at the existence of structures without allowing those structures to pre-exist the political possibilities available to critics. That is, once you've established what a social movement is "against", that which it is "against" is thought to pre-exist the social movement and movements always occur in a kind of dialectical relationship on an existing political plane. The movements, however much they oppose what exists/is hegemonic, nevertheless have some of their radical oppositional potential drained by defining themselves oppositionally rather than producing their own independently intelligible discourse of appeal.
To contextualize through criticism: one might think about how, for example, how ineffective on a large scale pro-socialism discourses in America are today. This is not because there is anything inherently unpersuasive about socialism: indeed, America has a long and glorious history of federally and state enacted "socialist" policies. It is, instead, because the American political imaginary coheres through an understanding of socialism that displaces and marks its discourses as alien to discourses of "Americanness" in the abstract. We can think of this as needing a kind of rhetorical means-testing: until a socialist policy is calcified in the realm of the political the presumption lies against its normative desireability. All of this operates under the presumption that something like "Americanness" exists, that it is known, and that it does a kind of natural/guaranteed work in producing a coherent political space.
Back to Zarefsky: how can we even begin to claim to know the contours of political space before we investigate it? I think that we cannot. All we can do is to inquire (following Foucault) after the conditions of discourse's intelligible enunciation. To do otherwise risks stipulating in advance that a certain something already exists as the antecedent to the discourses that exist, and in so-doing we might naturalize the existences of the very hegemonic discourses to which we are opposed. If we map progressive political content onto the form of social movements generically, we are left always doing criticism that produces social movements as outside an already established system. Once they are outside, there is a kind of historicized presumption against them. McGee was, as always, onto something like this before most with his seminal work on social movements. For now, careful of form and content!