There are different kinds of causes here. Clearly, the conservative demagogues radicalized their base. But the Democrats gave them the motive and the opportunity. The demagogues are the means or instruments of the radicalization (what Aristotle called the efficient causes).

Democratic centrism gave the Republicans no other way of flourishing in the two-party system, but to focus on purifying their brand. Demagoguery is the means by which Republicans purified (deranged) and distinguished themselves in a political system that shuts all the other doors.