Citizens United would be a decent candidate. Once it was established that donations were protected political speech, it effectively legalized bribery, and made oligarchy essentially inevitable. Most of the missteps since then have been motivated by folks trying to simultaneously play to populist talking points but also placate billionaire donors. The left needed an actual positive message, like the kind Bernie Sanders was pushing, that would energize folks and unite the overeducated with the working class, but that was never going to be acceptable to the donor class, and so candidates like him always had to be shoved aside for someone who would clearly cater to corporate needs. And someone who would clearly cater to corporate needs was always going to be a really tough sell and not really a solution to the needs of the moment.

That doesn’t really account for the rise of the tech bro fascist accelerationists like Mencius Moldbug and the Dark Enlightenment, which is a big part of the current moment and accounts for how the far right was able to hoodwink some billionaires into voting for a social collapse that seems very likely to hurt them also. But Citizens United still seems like a fair candidate for a point at which some of the last paths away from this outcome were foreclosed.