I doubt it.
If "destroy" means "kill," then it's a death threat and is illegal. But is that what SPLC meant and can a prosecutor prove it?
I don't think so. You can't kill an organization, so that's obviously not what SPLC meant. SPLC can argue that since it doesn't mean kill in THAT context, it doesn't mean kill with regard to people either.
In their defense, what they'll argue is that it's not illegal to say they intend to "destroy" a politician. It only means "ruin," and that's not illegal.