Little of both. Ronald Reagan tilled the soil, and the later Republicans fertilized it. Why wouldn't foreign disinformation groups sow seeds in such perfect fields?
A big chunk of the problem in US politics is the two party system enforced by first past the post. Very few people actually agree with 100% of either party's policies, so the deciding factor for most people becomes either which party do I agree with more of their policies, or in some cases which policies do I feel are most critical and therefore drive the overall decision. This has been made even worse by Republicans strategically picking policies to try to drive a wedge in between them and Democrats particularly around certain hot button issues like abortion, gun control, and religion.
He was very popular which made him that much more dangerous. His trickle down economics lie has done more to destroy the US economy than just about anything else. To this day there's still people who feel it's a viable model despite nearly half a century now of showing it doesn't work. He and Nancy Reagan are also responsible for continuing the culture war against minorities that Nixon started.
We don't know what we're doing, we hate each other, we can't agree on anything, and we love killing one another. I think if they try to divide us any further we'll just end up wrapping back around.