The following is an attempt at pulling the “disagreement” thread (as against the “respect for conscience” thread) of political liberalism. And doing so at what I take to be a general, fundamental starting point. Let’s see what I’ve got!
(I) When a group of people are trying to come to a collective decision together, they often aim, not at making the objectively (or rationally) best decision but rather at making the objectively (or rationally) best decision that is also acceptable to all (except those who are wicked or foolish in a way that is relevant to the decision at hand, acceptability thus being qualified acceptability or acceptability for qualified individuals). In this way, we often aim not at what is (objectively or rationally) best but rather at what is “consensus-best.”
This kind of case might instantiate the most basic way in which standards of qualified acceptability figure into collective deliberation. The essential thing seems to be that we are seeking as good a consensus as we can get (good on the objective or rational merits) in the face of reasonable disagreement regarding which decision is (objectively or rationally) best. We would not face this “problem of disagreement” if the relevant evidence were clear and accessible to all and clearly singled out some option as objectively (or rationally) best.
(II) Often in such cases of need for consensus in the face of dispute among reasonable-enough parties, we disagree not only about which option is objectively (or rationally best but about which reasons objectively (or rationally) favor some option and how much. And so I might give you a reason that I take to favor option A over option B (and that really does favor A over B) that is qualifiedly unacceptable to you (it is not supported by your reasonable-enough prior beliefs, values, etc.). And as objectively (or rationally) best conclusions might be irrelevant to which conclusion is consensus-best (due to being qualifiedly unacceptable), so good merit-favoring reasons that are qualifiedly unacceptable play no role in promoting or producing consensus-best conclusion.
(III) So much for the teleology of collective deliberation toward consensus in the face of disagreement. If the participants are hooked into or participate in this teleology in the right way, and I think they are or do, then we also get the following normative result. The agents participating in such a collective endeavor have (a) strong reason not to push unacceptable conclusions on others (even if they are objectively or rationally best) and (b) strong reason not to give reasons to others that are unacceptable to them (even if they are good reasons, relative to underlying objective or rational merit).
(IV) Relevant to the claim that political liberalism is a thesis about political justification per se, another thing we get from this picture is the following upshot for private reasoning: when we are (privately) reasoning about which decision is consensus-best, reasons that speak to objective (or rational) merit, if they are qualifiedly unacceptable, have no weight (or their weight is nullified or negated by their being unacceptable). In contrast, qualifiedly acceptable reasons that speak to objective (or rational) merit are also reasons that speak to which option is consensus-best. We get this structure only because the topic is what is consensus-best – about other topics the approach here would not generate this kind of result. I suspect that this is the only fundamental way in which the teleology of seeking consensus amidst rational disagreement impacts the structure of evidence and private, cognitive reasoning.
(V) How might such a story begin to make solid contact with the political liberal claim about political justification per se? Start with this: this sort of impact of unacceptability on private reasoning (about which option is consensus-best) would be inherited by any proposition that is made true by facts about options being consensus-best. So we might make our contact with the following proposal: (i) state S being permitted to coercively impose law L is constituted by (ii) the consensus-best option (between standing ready to allow S to coercively impose L and not) being to stand ready to allow S to coercively impose L. If so then any (qualifiedly) unacceptable reasons favoring the objective (or rational) merit of standing ready to allow S to coercively impose L would be irrelevant not only to [ii] but also to [i]. By contrast, qualifiedly acceptable reasons favoring objective (or rational) merit would provide support for both [i] and [ii]. I’m not sure how plausible this proposal is either substantively or as a friendly suggestion for political liberals. But it is one way, perhaps the only way, of getting the relevant sort of surprising and profound results about political justification per se, starting mainly from how collective deliberation that seeks consensus amidst disagreement has to go.
(VI) It does occur to me that convictions – beliefs of conscience – might generate a special or an especially strong sort of unacceptability. Maybe political liberalism makes essential appeal to this as well as to what is required to reach consensus amidst disagreement in collective deliberation. The full picture would also have to bring in other relevant circumstances (a lot being at stake, no easy exit from the cooperative endeavor, etc.) even if something like what I have laid out here is the most fundamental element. And the picture here, even at this level of generality, is probably still missing some important elements.