You're trying to axiomatically derive election law here. Every word in the statute matters. It is very probably perfectly lawful for SPLC to work to censor ideologies, even up to the point where those ideologies are coterminous with party definitions. The whole edifice of Citizens United is based on a sharp divide between advocacy and campaign finance; the argument you're putting forward is disfavored.

(That's not to say SPLC couldn't have fucked up and crossed the line, just that the general description given upthread of what they were doing did not in fact describe a violation of law).