I think that while it may seem like a slippery slope at first, it is what we need. I think it's actually quite similar to the immune system analogy in the comment above. We need to have democratic participants in a democratic process. Our immune system needs to repel the undemocratic elements in some capacity.

Admittedly the question becomes how we can in a fair manner judge what ideas in which context can have such a dangerous impact as to be non-democratic.

We all have to reflect how close fascism and other kinds of ideologies always are just beneath the surface, and that we are all susceptible of falling on that train it isn't specifically another group that is.