More broadly, I'm reminded of Stephen Resnick's story of his fellow MIT grad student Stephen Hymer going in to
Liberals and radicals do disagree over ultimate ends - more stuff, more equitably distributed, for them; the full and free development of human capacities, for us. But the more salient disagreement, at least in the current conjuncture, is over means. Liberals believe that the political process is ultimately a form of rational debate, in which the objectively best ideas win out and are then executed by a neutral administrative mechanism. Political engagement means situating yourself within shouting distance of the seat of power, and then making the case that your preferred policy is in the best interests of everyone. Who you are doesn't matter, just the merits of your views. Carl Schmitt, interestingly enough, gives one of the clearest statements of this conception of politics:
All specifically parliamentary arrangements and norms receive their meaning first through discussion and openness. This is especially true of the fundamental principle that the representative is independent of his constituents and party... The characteristic of all representative constitutions ... is that laws arise out of a conflict of opinions, not out of a struggle of interests. ... Conduct that is not concerned with discovering what is rationally correct, but with calculating particular interests and the chances of winning and with carrying these through according to one's own interests is ... not discussion in the specific sense.Schmitt, the anti-liberal, saw better than liberals that this mode of politics is specific to the particular institutional context of parliamentarianism. A context that remains very important, of course, outside of the formal political domain as well as within it.  But it's not universal, and in particular it can't be the last word in a society that is divided by fundamentally conflicting interests.
Radicals, by contrast, see the conflict of interests as fundamental. Or rather, we see it as inescapable in politics as long as it exists in economic life and society generally. From this point of view, arguments are won in parliament only thanks to the rioters, literal or figurative, in the streets outside. And liberalism as a concrete political project is a compromise between opposing interests, one that's always open to renegotiation when the balance of forces changes. So unlike in science (liberalism's implicit ideal), progress is always reversible, so no political struggle is ever definitely finished. Any given compromise is only sustainable to the extent that there are social forces striving for a horizon beyond it.
 For example, I'm current serving on a university hiring committee, and the norm that discussions must be conducted only in terms of differing opinions, never opposing interests, is very strongly felt.