David Kopel writes:
My Independence Institute colleague Rob Natelson examines the question from an originalist perspective, in a new working paper published on SSRN. His analysis is summarized in this blog post on his website. In brief: political contributions are best analyzed as a form of Freedom of the Press. The Freedom of the Press includes the right to anonymous authorship. The right can be breached in cases of abuse, as when a civil libel plaintiff needs to discover the identity of the person who libeled him.
Read more on The Volokh Conspiracy.