Starting with Ronald Reagan's presidency, the US government willingly decided to ignore the anti-trust laws so that corporations would have free rein to set up monopolies. With each successive president the monopolistic concentration of business and shareholding in America has grown precipitously eventually to reach the monstrous levels of the present day.
Today's level of monopolistic concentration is of such unprecedented levels that we may without hesitation designate the US economy as a giant oligopoly. From economic power follows political power, therefore the economic oligopoly translates into a political oligarchy. (It seems, though, that the transformation has rather gone the other way around, a ferocious set of oligarchs have consolidated their economic and political power beginning from the turn of the twentieth century).