Does anyone have reference to a sociological analysis on why a two party system (no spoilers about why it’s not, please) seem, in my non analytical perspective, to regularly tend towards very close splits between the two main sides?

I think there has to be a self corrective feedback going on. It could be the wife pissed off at the husband’s rants, who then votes the opposite; or it could be the simple fact that the parties automatically focus their efforts on the undecideds (aka uninformeds) to compensate, with relative equal success, or something else.

Or perhaps it is simply that the parties themselves automatically temper their pleas to level the appeal, whether by lies or sincerity.

In any case it does seem to me that, however unpleasant the process, the alternative of extreme landslide wins by any one side is a formula for one party rule. So perhaps what we have is the best we should wish for?