Apropos of the present Frankfurt School/anti-semitism debate, here's an idea I'm working on: there are two main kinds of decline theories, degeneration theories and seduction theories.

Degeneration theories hold that decline is inevitable due to some fatal flaw at the origin of the regime/nation/civilization. As such, they oscillate between extremes of radicalism and pessimism. You either have to start again completely or you can't fix anything.

Seduction theories hold that decline is not inevitable, but was caused by the introduction of some alien idea/population/institution. The political task, therefore, is to root out the source of that corruption.

Because they focus on a particular bad influence, seduction theories are more politically actionable. But for they same reason they also encourage conspiracy theories and scapegoating.

I don't think there's any particular ideological affinity to these categories. In other words, both kinds of theory comes in "right" and "left" versions.

In fact, what's interesting about them is that they allow us to perceive "strange bedfellows" that wouldn't otherwise be grouped together.