The problem is not theoretical but _practical_: the U.S. proletariat are _too weak & disorganized_ to take the state power -- hence the _severe_ "constraints faced by Third World progressives" (the constraints which have become severer since the disappearance of the countervailing superpower that the USSR was). Hell, they seldom get to take power _at the levels of school boards & city councils_!
American leftists are often anti-statist, reflecting the decentralized structure of American polity; it seems to me that anti-statism of many American leftists should be read as an expression of political "sour grapes," so to speak. Most American leftists don't think that they will ever come into power, so they rationalize their projected "failure" ahead of time by theorizing the state power as what is to be avoided, for "power corrupts" in their minds.
Anti-statist European philosophers like Negri & Baudrillard tend to be big fans of things American, seeing paradoxical virtues in the weakness and disorganization of the working class here: "Against the common wisdom that the U.S. proletariat is weak because of its low party and union representation with respect to Europe and elsewhere, perhaps we should see it as strong for precisely those reasons" (Michael Hardt & Antonio Negri, _Empire_, Cambridge, MA: Harvard UP, 2000, p. 269). That's probably because they despise both hitherto existing political parties (Communist, Socialist, Trotskyist, Maoist, whatever) and potential political parties on the Left that may be born in the future _more_ than they abhor capitalism & imperialism....
Yoshie