Mark Kleiman's post reminds me of something I forgot to mention: It is illegal for foreign citizens to purchase campaign ads advocating for the election or defeat of a U.S. federal candidate. However, the Citizens United ruling doesn't distinguish between foreign and domestic corporations. Even if a future ruling did, domestic corporations can easily be owned 100% by foreign nationals. That means that Citizens United effectively opens up an avenue for foreign individuals and corporations to directly affect the outcome of individual elections.
(Via Eric Martin.)