Limits on speech are sometimes necessary to protect speech.
If you have limits on speech, then you no longer have free speech, so "to protect speech" is not a valid justification for such action. Limits on speech are sometimes necessary to protect other rights.
To steelman your argument:
Limits on contributions in context of politics are necessary, because protection of democratic principles in a society is more important that any one's individual freedom of speech. That is, if you are using your immense wealth to corrupt "one person one vote" principle, then it is a question of rights of many individuals vs. rights of one individual. This doesn't change the fact that it is still censorship.
Still, I would like you to address how your arguments for social media corporations' right to censor political views on their platform is categorically different from logic used to pass Citizens united.