The United States was founded as a democratic republic, a compromise of sorts between monarchy and full on democracy which the founders feared would put too much power in the hands of the mob. Political power, and the vote were restricted to white males, and also by various economic tests. The vote has been progressively widened over most of the past two plus centuries, but various factors have narrowed political power, concentrating it in the hands of a tiny number of the super wealthy.
Since the Supreme Courts Citizens United decision, the American election system has become a system of legalized bribery. For the most part, the bribes don't flow directly into the pockets of candidate (New Jersey governor Chris Cristie being slightly exceptional, though not, of course, in New Jersey). Instead, the bribes are direct or indirect campaign contributions.
Economic and technological factors have made running for political office enormously expensive, and concentration of media ownership in the hands of a few giant corporations and individuals has made independent sources of election information rare. In addition, the rise of so-called "think tanks" that deal more in propaganda than thinking has provided a focus for oligarchs to organize and push their agendas.
The immense wealth concentrated in a few hands gives these people immense power to influence elections at every level. The two Koch brothers, whose joint wealth comes to $ 85 billion, have announced plans to raise over $880 million for the 2016 election. That number is far larger than the total amount of money on elections just a few years ago. That money, together with a plan, gives them great power to influence elections at every level.
For example, my State of New Mexico is traditionally narrowly divided between Republicans and Democrats, with a slight edge to Democrats. State House of Representatives elections are traditionally local events based on a lot of direct voter contact and very little campaign expenditure. My district traditionally features very close races and the Democrats had an excellent candidate this year, who probably had a slight edge until a late dark money smear campaign featuring full page ads in the local newspaper tipped the balance in favor of her opponent.
A few key interventions like this changed the State House from Democrat to Republican.
Of course not all oligarchs are as radically anti-democratic as the Koch brothers, but they are all oligarchs, and as such have a lot more in common with each other than with the rest of the country. Consequently, the opposition Democratic Party is also dominated by those heavily indebted to some group or other of the super wealthy.
It's not a pretty picture for those who value the Constitution.