The PACs. I think this practice should be considered blatant corruption in any democratic system as it enables large corporations and wealthy individuals to predetermine which candidate or party has even the slightest chance in elections. In my home country, of course, there are private political funds as well but those are not nearly as important in our system as there is solid public funding for political parties based on past election results. I might be wrong but I always thought that the insane amount of private money that fuels US elections boils down to the US being a plutocracy rather than a democracy.