I'd not have thought that this would be a controversial statement given the current nature of US politics. Both parties are highly dependent on enormously wealthy donors. It's hard to believe that these people are getting nothing in return for their money. Graham's surprisingly naive about this. He simply says "The great concentrations of wealth I see around me in Silicon Valley don't seem to be destroying democracy." I couldn't quite believe he said this after we've seen the complicity of Facebook, Google etc. in the surveillance state.
It's a bit like asking for a citation showing that Barack Obama is currently the president. It would be hard to know where to start. Someone already mentioned the Koch brothers, and the recent news is not difficult to Google.
In fairness, I've always seen the Koch brothers as a heartening example of the limitations to the ability of individuals to buy government. They had to give up on the Libertarian Party and can't buy the Republican caucus, never mind swing the election.
On the other hand, if it wasn't for the Koch brothers it's distinctly possible that few people would have heard of Austrian economics, and most of those would be academics writing it off as of purely historical interest.
I would not discount the influence of their lobbying efforts.
The Koch's helped to drag the debate so far to the right that even the Democrats look like a conservative party.
Perhaps their money won't buy them the party they want - but it certainly will kibosh any talk of significant tax increases on the wealthy, or single payer health care.
(disclaimer: I am from Canada - so an outside observer of US politics).