Today on NPR Nina Totenberg gave her usual excellent report on a hearing before the Supreme Court. This one was in Citizens United v FEC. One of the colloquys she mentioned was between petitioner's counsel Ted Olson (Theodore F) and the Justices concerning whether corporate 'persons' should have the same rights as natural persons. As far as I could tell, *nobody* - not current Solicitor General Kagan, not any of the other Justices - pointed out that corporations do NOT have the right to VOTE. The right to vote is definitely a right reserved only to natural persons. It makes sense, then, to restrict to natural persons the right to contribute to the campaigns of the candidates for whom only natural persons can vote.
I checked the briefs very quickly, and it looked like this key fact was not made explicit by any of the amici, either, although the League of Woman Voters did mention the "bedrock equality principle of one person, one vote." Which does not apply to corporate "persons."