The founders of America for their part understood their government as a republic, not a democracy. They had varying views on democracy, almost always negative and qualified by some stake in the country. The movement to universal (white) suffrage was largely about shoring up support for the regime, because early America was on shaky ground and relied on militias to conduct its wars. Men with guns can only be pushed around so much, and so far as Americans conceived of "democracy", it existed because enough of the people had guns to defend their stake, and the modern professional army was not yet a thing. You had people of fairly humble origins rising to prominent positions, and the founders themselves were not marked as special or carried any particular distinction. You had some real lowlives become respectable enough men. The American ruling class considered themselves an aristocracy and put on airs like they were such, but it was a "natural aristocracy". None of these men had extensive pedigrees to say they were awesome, and it was an aristocracy that a few people could enter if they were ambitious enough.
That was the intent behind nominally democratic polities, whether they were ancient or modern - that someone could in theory rise from nothing to become something. The Athenians weren't giving people office out of some charity or because it was nice, but because it was a way of supporting the basis for the Athenian army. The actual decision making was not the vaunted task it is made out to be, and that has been known all along. Athens was not a communist city-state or anything close to that.
Anyway, the myth of "liberal democracy" is a much more recent invention. Ask most Americans in the 19th century and they would ask you what a "democracy" is supposed to be, or they understood democracy as merely an idea or suggestion, and one that had little relevance to their lives. The deal to keep the peace in America is that it was a big country and you could piss off and do whatever if you had money, and the government had a deal to leave alone that which shouldn't be fucked with. It didn't work that way for the slaves or the tribes to be removed, and it didn't work that way if someone ran afoul of capitalism and didn't have money. America is a country of scamming and grifting going back to its colonial past. Liberal "democracy" isn't even the farce that representative democracy was. The liberal democracy was premised on the idea that the masses don't actually think, and that ordinary people are philosophical zombies. The elections might as well be fake, for if anyone were to "vote incorrectly", the system would discipline the would-be rabblerouser and reinforce the belief that you can't change the system. Then the voters would return and be given the choice of voting correctly, i.e. voting for the candidate that was pre-approved and given proper PR backing. There are very few truly competitive elections in America, and where the elections are competitive, the candidates always move to the imagined "center" even though nobody fucking wants the center. There is at most an inertia of the people to keep things mostly as they are, because those who still vote are those who approve of the system. Anyone truly oppositional realized a long time ago that liberal democracy is rigged and the real power moved to CIA and friends anyway. There were machines throughout American history, but the machine of "liberal democracy" was a fully technocratic system in its honest conception, and by the 1950s, no one who knew what was what actually believed you could vote to change anything. It was accepted that the institutions were above electoral politics, and the politicians only existed to grease the wheels. The past republican government did entail voting for actual people who ostensibly directed the bureaucracy, though you still could only vote for pre-approved candidates.
A basic rule of politics is that everyone lies about everything, including the nature of their project. Imagining that institutions can be made good on a permanent basis is a folly of republicanism, and that idea goes back to the antidemocratic movement in Greece and the ideas the Romans had to shelter an aristocracy. For the Romans, there were no bones made about it. They knew they were ruled by a nobility and the plebs were there to make something out of themselves if they could, while the patricians LARPed as the gods and had their orgies. That's all it was really for, the orgies and drunken parties. It is the same with America, and it was in the end the same with the Greeks except there was more infighting because Greeks love to argue about shit.
For most of history, the vote was recognized as a joke, with the working class usually denied even nominal participation. The liberal justification for letting the workers vote at all was to induce them to "vote correctly" and support candidates that would advance the liberal project. The conservatives in Britain could only win because the liberals were that fucking foul and disgusting; but the typical voter, and this is true anywhere in the world, is not invested in ideology. Ideology drives only a certain type of person, or appeals to certain interests and uses a lot of lying to make a lot out of nothing. Most people pretend democracy is legitimate - there never is any serious inquiry into whether the electoral process can even be trusted, and ballot box stuffing is an old practice. In liberal democracy, the rigging is thrown in your face, because liberal "democracy" is nothing but a cover for a scientific dictatorship.
I honestly don't see Marx favoring democracy in the sense you and I would have it. He only cared about people who got his system, and if you weren't able to get with it, you didn't have a right to expect anything. There is virtually no one that truly champions democracy, and when they do it is always qualified by the interests of the middle and upper classes. The low, as a rule, only want this onerous beast out of their life. Don't ask them how it will happen or how such a state can be maintained. They simply hate these assholes cajoling them and making a moral posture to defend something clearly immortal and depraved, that absolutely refuses to respond to what the lower class understand to be justice or even conditions allowing the lower class to live. Democracy has been understood as letting the poors have their shit and keep it, and that is the thing politicians avoid more than anything else. You cannot make political institutions into democratic ones. Democracy only ever exists in spite of the institutions, because there is a will in people that can resist such intrusions into their lives and their preferred social relations. Where Marx is writing about that, he calls for the free association of producers rather than political concepts of democracy, because the idea of working political institutions isn't really Marx's project. If it was, he would have written a concrete plan about what to do, instead of giving hints (because you were supposed to get the hint that none of this was about the workers, and the workers would be told to be grateful if they receive anything at all).
A democratic movement today would be thoroughly anti-eugenics and would be able to offer a principled defense against it, instead of accepting the empty sops. If such a movement exists, it would find itself immediately in conflict with all of the dominant ideas today, and any such movement would be seen as a transgression of the instituitons. Liberals, communists, fascists, all agree on this. No one can challenge eugenics, and no one can challenge the Satan. At this point, we are so far removed from any democratic idea that it might as well be like discussing unicorns. The people are effectively defeated and displaced by institutions that lord over them imperiously, and this has been accepted on faith because enough people are beholden to those institutions, even when those institutions necessitate permanent intercine conflict. There is this naive faith that the institutions will save you, but the institutions are set up for the benefit of particular interests. They do not want us and never did. Democracy as a force has always been premised on questioning any institution that claims authority of any sort over actual people. This was displaced with an anarchist version which only dealt with the pretenses, while defending the most predatory individuals that were the most strident opponents of democracy one could find. Democracy would have entailed the lower class forming their own institutions and declaring independence from them. So far as it is raised as a prospect, it is compared not to middle class revolution but the ruin of the contending classes - because without the lower class, the other classes have nothing to lord over, and the project would be exposed as the farce it always was. Democracy would entail the people taking their shit back and never allowing what happened in the 20th century to happen again. We don't have any reliable way to enforce that even in the short term. In the 21st century, this is even more distant, as the psychological assault compromises the very individuals that comprise "the people" as an independent force. The idea that democracy involves subordination to an imagined collective is the first fascist corruption of a historical reality. It was not at all how anyone envisioned democracy in any sense, because democracy wasn't envisioned as something that was institutional, but something that would have created its own understanding and would have been responsive to self-criticism. There would have been a peaceful path to reform by actually speaking to people, instead of cajoling them to think the correct thoughts. The latter is what a republic does by design. It's in the republican idea.
How you would build a functioning democracy would require many steps that are nowhere near happening. It would be nothing short of a religious transformation, and very likely the form a real democracy would take is a new religion altogether, that explicitly counteracts the "religion of science" that was the veneer the scientific dictatorship and oligarchy chose. Imagine something like Islam but with a whole new philosophical take that ran counter to basically everything we take for granted as "The Science" and our concepts of what it means to rule. Islam itself is wholly unsuitable, but the concept of the ummah is an interesting model of how a supranational religious community would be understood. So basically, imagine a version of Islam that was communist and not obsessed with making everyone submit through weirdass rituals, but was willing to be as militant as necessary to defend the practices of the religion. Eugenics did nothing short of establish a new world religion - or rather, revived ancient Satanic practices and glorified them, stripping them of their historical context and making them into a pseudoscience.
Realistically though, if you want to defend what counts as democracy, you would focus of letting people have their shit back, and stop trying to cajole them to be what you would prefer them to be. People can make their own decisions, and if you are unwilling to meet them at all, then why should they follow you? You can only resort to an external threat for so long to get people to support you, and the idea that you are owed service to an alien "society" is not how any functional socialism could operate. None of the actual socialist countries made a fascistic claim that you were entirely the property of "society" in this way. The socialist countries didn't have the liberal conception of legal rights, but like any practical government, they understood people were people and had to be treated as such if you wanted anything out of them. The cajoling and hectoring is something you don't even do to slaves. It's something you do to people you want to kill or sacrifice.