Here, at SCOTUSBlog:
Many are already raising the specter of the McCutcheon case being another Citizens United, this time dramatically changing the basic law on contribution limits. But McCutcheon and the RNC are not making that argument and are not challenging the validity of the base limits on contributions to candidates or parties. They think they can win within the traditional Buckley framework that permits contribution regulation, but only if properly justified. And Justice Kennedy in Citizens United went out of the way to say that the case involved independent expenditures only, with no direct impact on the validity of contribution limits. But one of the linchpins of his decision was that the only compelling interest that justifies campaign finance limitations is preventing direct quid pro quo corruption. To the extent the supporters of the law seem to be claiming that contributors like McCutcheon might have greater access to and influence on Republican elected officials, these days that does not seem to be much of a winning argument in the Supreme Court.