To me it appeared that the failed attempts to ban NPD in the past actually helped them gain more supporters.
What makes AfD stronger for sure is just going “lol nah we’re not even going to try”:
https://www.tagesschau.de/inland/innenpolitik/afd-verbot-antrag-100.html
If they don’t try, then it means that “it can’t be that bad, it’s just a normal party”, right? 😡