Honestly I think I mostly wealth inequality, the internet, modern media and modern culture. And I don't think those are necessarily down to things like division, people have been strongly divided since we were apes but I think there are some particular wedges that politicians have figured out work better in this current climate. One of the most obvious ones seems to be populism fueled weirdly by the exact thing it's against.
We're all mad at these billionaires who are after power, these politicians who are trying to become billionaires though power and that is one thing is seems everyone can get behind but somehow those very people are the ones we elect because they claim to be for the people and want to stop those abuses. They figured out they can use media fuel this type of fire really well. It just so happens that all sides and I don't mean just in the US have figured this out and are good enough at lying (or honestly the populations is bad enough at detecting lies, perhaps due to the short attention span of modern culture idk).
The complexity of wealth inequality is not something I think we can really get into but it would be nice if we as a population could get a little better and not buying the obvious lies that they are somehow actually for the people and against elites with zero evidice to support that.
To get maybe more granular is seems much of the modern US democratic party's identity is built on being anti-eltie and for the people and then orange man came along and also went for that play except people were fed up with the dems promising that and never delivering and so here we are with another old man with a red hat instead of a blue hat that is also not delivering but he's not delivering with about 200% more chaos.
Also populism is obviously not new but extreme versions of it come in waves throughout history it seems when conditions are right and I think the internet among other factors has created those conditions in various ways.