This has been answered very nicely by Paul Krugman, Michael Kinsley, James K. Galbraith and many others on the liberal and progressive end of the spectrum. The US is already a de facto oligarchy. Clinton, Bush and now Obama have worked diligently to formalize American Oligarchy, with the unalloyed support of Wall Street and a huge percentage of corporate boards, Democrats, Republicans and American super-rich.
I emphasize that this issue has been expressly raised by many who are far less left-leaning than I am.
It's a possibly-encouraging feature of American democracy that some who could easily ride the wave among fellow oligarchs are vocally opposed to the decline (among them, George Soros).
By contrast, a deeply discouraging feature is the absolute failure of the vast majority of American journalists to do anything even remotely resembling the work they assert they do. Here is London Times editor Robert Lowe in 1851:
The first duty of the press is to obtain the earliest and most correct intelligence of the events of the time, and instantly, by disclosing them, to make them the common property of the nation... The Press lives by disclosures... For us, with whom publicity and truth are the air and light of existence, there can be no greater disgrace than to recoil from the frank and accurate disclosure of facts as they are. We are bound to tell the truth as we find it, without fear of consequences – to lend no convenient shelter to acts of injustice and oppression, but to consign them at once to the judgment of the world.
Similar thoughts have been voiced by H. L. Mencken, Mark Twain, Studs Terkel, and others. But today, we hear prominent members of the American news establishment explicitly reject this journalistic duty. The torrent of condemnation of and vile misreporting on Julian Assange is a perfect example of this (by, among others, the New York Times, including 'star reporter' John Burns).
Equally discouraging is the deeply delusional state of the American people. Economist Ken Rogoff was recently on Charlie Rose (Rose usually dismally middle-of-the-road to conservative). In response to a question on why Americans support the Republican campaign to cut taxes for millionaires and billionaires, Rogoff — absolutely on-target — said something to the effect, "Because everyone expects to be rich." I've heard this on the street myself — people with no prospects of any kind asserting with total confidence that they are going to win the lottery. I heard this twenty years ago in Massachusetts when it was reported, during widespread opposition to a modest tax increase proposal, that many lottery ticket buyers spent more on the lottery each year than they paid in their state taxes. (Dollar for dollar, which do you think returned more value to them? Hint: Expected gain on a one dollar lottery ticket is less than a penny.)
The US may very well have the most ill-informed, poorly-educated, deluded population of any country on Earth. (But China seems determined to best us on that count.)