Can you explain why it is correct to call American Liberals "Liberals" and not Social Democrats when they are predominantly Social Democrats? The only "Liberal" thing about them seems to be their heritage, but that smells an awful lot like calling British Labour "Socialists" because they used to be Socialist decades ago.
I suppose it is possible to be a "socially conservative" Social Democrat i.e. you advocate the redistribution of wealth and the creation of an American NHS but you disagree with drug legalisation and equal marriage, so American Liberals are "socially liberal" Social Democrats in that sense.
Well, firstly, they aren't predominantly Social Democrats. The American Left you see on the internet might give you that impression, but that's because we've got the whole "young radicals" thing going for us. It's a very Big Tent situation. Same with the right. The theocrats don't
really give a shit about what the robber barons want, they just have non-conflicting interests, so an alliance is logical.
I'll take your example of an American NHS. Certainly, there are some people who want that (but not many). There are also ones who just want the private insurance corporations to become public insurance corporations, ones who want them to only be partially regulated (these are the ones who like Obamacare), ones who want to let them be unregulated, and ones who think being poor is evil and should warrant being denied medical care (and something something free market, punishing poor people is the important part here).
So the Poor Killers and the Market Worshipers get together and try to force the issue because it gets them both what they want, while the Universals, Nationalizers, and Regulators try to work out something they can all agree on so the Poor Killer-Market Worshiper Alliance doesn't build up enough populist rage at poor people and government workers that they get their way. And that is how you create Obamacare.
It's not like Labour because we altered the definition of Liberal. What I describe is what virtually every American thinks of when the word Liberal is used. The fusion has existed for so long it isn't going to come apart for the foreseeable future. So, by definition, in American political discourse, when one talks about "them damn Liberals ruining the country", they are referring to what exists in a factual sense. Conversely, Labour just wants to use the language of socialism while getting the benefits of centrism. Your PoliSci definitions do not take predominance over those elsewhere in the world.