It is not exactly unsubstantiated: the powerful people within the system have basically admitted it. See, for example, Chris Dodd's comments:
How exactly might you interpret a comment like, "Candidly, those who count on quote 'Hollywood' for support need to understand that this industry is watching very carefully who's going to stand up for them when their job is at stake?" Politicians count on big business for support and the leaders of those businesses make sure those politicians do what benefits them.
It's not just Hollywood. Obama needed the support of the pharmaceutical industry to push the healthcare bill through; the deal was that the Obama administration would fight harder against medical marijuana and imported drugs, and despite the fact that Obama's public statements included talk of depriotizing marijuana, his first two years saw more raids on medical marijuana dispensaries than all eight years of his predecessor. There is little doubt that the defense industry has a comfortable relationship with the government either: we spend as much on buying their weapons and systems as we spend on social security, and no president after Eisenhower has failed to involve the US in some military engagement or war (most of which were entirely unnecessary).
It is not that democracy is discredited, but that we question whether we even have a real democracy anymore. If your only choices are "far right" and "not so far right," does your vote really count? One has to look to the extreme minorities, third parties and independents, to find anything that could be called "left wing" by any reasonable standard.
I place the beginning of valid questions about American democracy in the 1970s. That was when the Democrats began catering to big business just like the Republicans, and it was in that decade that we (probably coincidentally) saw the beginning of paramilitary law enforcement. It was also the decade that saw the beginning of explosive growth in executive branch power (once the dust from the Nixon affair settled, anyway), which at this point has come to mean "when the president has it in for you, you're dead." As an example, it was in the 1970s that the attorney general's office gained the power to declare drugs to be illegal without having to wait for the wheels of democracy, and to arrest and prosecute people for possessing those drugs.
So can you really blame us for questioning the democracy of the United States? It seems like the only democratic processes that matter anymore are those that decide which set of big businesses will receive help from the government.