While I don't necessarily disagree with your gist here, I don't think it is accurate to characterize the ACA as "of the left". It was just Romneycare expanded to the Federal level, the "left" couldn't even get the "public option" included in the final bill. The whole thing was successful in that it got health insurance to a whole lot of people who didn't have it before, an outcome supported by many or even most on the left, but the actual ACA isn't really something leftists wanted or genuinely support as anything more than a stopgap on the path to actual reform. If the ACA actually did things to drive rent seeking behavior out of the health care industry and guarantee universal access it would be a great example of the phenomenon you are describing. The actual ACA is a much better example of the Republicans (and the the health care industry) running circles around the Dems during that era than of the Dems implementing leftist policy,

Taking that attitude you're showing you're not here in good faith.

If you want good faith arguments perhaps don't start with condescending comments about "kindergarten" level civics and have enough charity to at least start from the premise that it might actually be you who's wrong rather than just assuming that any argument you don't agree with must be the result of your interlocutor being kindergarten level dumb.

You construct a twisted, narrow interpretation so that you can walk out on it and say look at this very narrow interpretation, explain this narrow interpretation.

Followed by...

Do they lose? Do they win?

Broad and wide-ranging narrative ... anyone?

And then you go on to tell a story about what each president 'would have done' which, I presume you must have gained from direct personal conversations with them, unless... Oh, you're not just believing things they tell the newspapers... You sweet summer child...

what would happen if they won consistently and overwhelmingly? They'd move left. They could do left things, without losing the next election. This is pretty simple.

It might seem simple to you. But it contains two hidden premises and two logical flaws.

The first hidden premise is that they actually want to move left (and so would take an opportunity to do so). You've not yet made a case that they do. A scattering of slightly-left-of-neocon policies is not very convincing.

The second premise is that each event is a response to the last and not to any of the hundred other factors in American politics at the time. Again, just showing one thing followed another does not prove it was caused by it.

The first logical flaw is that you've still not provided a mechanism by which successive democratic campaign teams know somehow why they lost, that it was their slightly leftist policies and not, again one of the other hundred factors in politics at the time.

The second logical flaw is that you've still not explained why democrats need an actual election to find out that lots of leftists will vote for them. Why can't they just poll, like everyone else does? They presumably rely on polls to tell them what policies these non-voters want, so why do they need an actual election victory to learn that in four year's time these people will likely vote for them. Why can't they just ask? That's the normal way all other political strategies are worked out - focus groups, polls, town meetings... You're singling out willingness to vote as a fact about potential voters which is somehow inaccessible to the democrat strategists without the proof of an actual election win, but assuming other facts, like the policies they'd like, can be ascertained. Why?

this is basically just a long ass history of saying that even when they give the left policy, the left never shows up, so it wouldn't make any sense basically ever in the future for them to implement any leftist policy at any future point. we have to assume that they're rational actors rather than idiots, and that they're going to keep doing the thing that, according to you, makes them succeed. there's no amount of local grassroots "just vote harder" -ing that is suddenly going to get those left wing voters to suddenly pop up if there isn't any left wing policy proposals. so the democrats don't go left because they won't win, and the left wing voters don't show up because there's no left wing policy.

I also like the idea that actual populist, left wing policy implementations only ever cost them, only ever make them lose votes. also, this idea that they'd move left suddenly if they started winning, just, basically for no reason they'd start moving left, is awesome. very cool. by what mechanism would they move left? why? by what mechanism would the left actually have any leverage over them, in that circumstance? sure, they'd lose the votes, but then they'd actually have to implement left wing policy, which means they would totally be fucking over their much more important corporate and media sponsorships, and they'd also be basically eviscerating their own political power. you can see this in the very simple example of "no democrat will ever change the fptp system", because then they would stop getting elected, because they benefit from that being the voting system. this is basically the same principle by which they won't, say, do massive amounts of housing ownership reform.

also, what's your opinion on serious housing reform? what do you think about that, what do you think about, say, eliminating massive rental companies, or nationalizing them, constructing a large amount of housing, and then providing it for free to people, thus making them less dependent on their job and more secure in order to take risks on, say, doing actual forms of political activism? what do you think about the same being done for healthcare? employment? can you see any reason why the people who are currently in power might not want any of that to actually occur? can you think of any possible reason why those people which are currently in power might not want that shit to exist precisely because they have been selected by those systems as a product of their reinforcement of those systems? to break it down more, perhaps, why do you think CEOs tend to be incompetent assholes? is it just because of some like, cosmic trick, or is there perhaps a system there that's going to reward incompetent assholes over people who actually have beliefs?

also, I find it funny how you're accusing the guy you're talking to of having a narrow interpretation of history, that they're construing everything to work around, but then you're also turning around and saying "it's just so simple: the left never shows up, so the democrats will never go to the left!", and then retroactively giving an incredibly simplified and narrow retelling of history in order to support your point. any mention of the absolute slew of right wing legislation, that any of these people have pushed, which might be a reason why the left might not be showing up for them?