Let's start here: I don't know what you mean by "moral mechanism" when you say "Utilitarianism as the moral mechanism of social policy".
I'm pretty sure you don't mean "moral mechanism" as used in this article: Davenport, D. Moral Mechanisms. Philos. Technol. 27, 47–60 (2014). https://doi.org/10.1007/s13347-013-0147-2
Regarding de Tocqueville:
> "Democratic laws generally tend to promote the welfare of the greatest possible number; for they emanate from the majority of the citizens, who are subject to error, but who cannot have an interest opposed to their own advantage."
I understand the central argument of the quote. However, I don't think it demonstrates a strong enough connection between utilitarianism and social policy to call it "the" moral mechanism, granting of course, I'm not exactly sure what you mean (see above), but I am thinking it is along the lines of "moral influence" or "moral foundation".
Also, the claim that citizens cannot have an interest opposed to their own advantage is, at least, overly simplistic. Yes, de Tocqueville qualifies it with "Democratic laws tend", but I don't think this is enough of a caveat. Correct me if I'm wrong, but he didn't didn't do an empirical study of legislation nor did he attempt to categorize the effects in a utilitarian framework.
So my responses to dT's claim: First, preference aggregation is non-obvious at best and quite controversial in practice. Second, we have to talk about time scales. Some policies are intended to have long-term benefits with short-term costs, for example. Third, those with political power have the ability to use it in ways that disadvantage others. How often this happens would require a proper research design, which (correct me if I'm wrong) dT did not do. I view his work as valuable but largely as a record of his observations while travelling. He was certainly not a neutral observer.
I'll add a different point: given the phrase "Utilitarianism as the moral mechanism of social policy", do you view it as a metaphor? In what sense are you using it -- trying to identify patterns through history?
When I read philosophy, I actively seek to contextualize it. For example, I ask:
(a) Are the terms clear (admitting it can take work to get the context);
(b) Is it a way of thinking about the world (i.e. a reframing)?;
(c) Does it offer explanatory power?;
(d) Does it offer predictive power?
Another way to capture these four questions is: "To what degree is a particular clear, coherent, and/or interpretable by others? How does it assist in thinking about the issue? Is it testable in some way, not necessarily through a strict scientific method?"
Hopefully this background gives you some context as to why I'm digging into this phrase. For the sake of argument, let me say (and let you respond) that all four lettered questions are hazy to me.
Ok, now a few remarks about the United States and its social policies over time:
In the US, I'd like to highlight some contributing factors to "how we got here" with regards to social policy: (1) the historical moment; namely, perceived injustice by the colonies inflicted by the British; (2) the influence of common law on US legal foundations; (3) the innovation, so to speak, of the US form of government; (4) a long, complex history of social policy in the US, which includes religious influences, shifting awareness and beliefs, a civil war, mobilization of voters, and ensuing legislation, waves of resistance to change, and so on.