Richard McGregor in the Financial Times there are three ways of funding US Presidential candidates. Both donations, capped to $40000, and uncapped super-political action committees, must be disclosed. But "non-profit social welfare bodies... can take as much money as they want, anonymously. These are supposed to operate exclusively for social welfare reasons, but everyone knows their sole purpose is to win elections" (quoted from MoneyWeek 6/7/12).
The problem of hidden vested interests buying political support has the potential of corrupting democracy. If individuals vote on each issue, as I propose in Interactive Democracy, the problem becomes far less.