"Corruption" means something. It means more than just incompetence. It means more than just government acting in a way you don't like. It means more than just procedurally awful elections. Corruption means that the state truly ignores the rules, or behaves truly arbitrarily. It's rent-seeking behavior, it's outright participation by authorities in criminal enterprises, it's open threats for bribes.
I know this sounds like a semantic or pedantic distinction, but it's not. Throughout the vast majority of human history, people have had to deal with the arbitrary whims of local authorities: police, militias, barons and dukes, nobility of all kinds, bureaucrats, investigators and religious authorities. Bribes were near-ubiquitous. Patronage, electoral machines engaging in out-and-out election fraud, etc. were a major part of U.S. society. I suspect that if you pointed most people to what we have in the United States and asked them if it it was corrupt, they would laugh.
As in so many other areas, what we have today is the best we've ever gotten. By and large, most Western countries have pretty responsive, capable bureaucracies and civil services.
I say this as a Bernie supporter, aware of the obvious conflicts of interest caused by massive donor money, and of the massive election fraud perpetrated against Democrats in 2000 and 2004 and against Sanders supporters in 2016. Yes, America has a serious issue with police brutality and with cops treating communities of color like an occupying army. Yes, the primary system has had major irregularities. But even here, in many cases, the police aren't being corrupt: they aren't haphazardly, arbitrarily or criminally enforcing and following the law. Rather, they're operating within bureaucracies that are being given directives by the public, bureaucrats, media, etc. and are following those rules. Similarly, corporations may engage in some nasty behavior like Teflon's dumping of toxic chemicals, and we see bad behavior like Michigan and Flint's poisoning of their citizens. But, again, these tend to be either done within the law or be individual bad actors. Even the Panama Papers don't really change this picture.
So what about actual, bona fide corruption in the United States?
Transparency International, the leading source on corruption issues globally, ranks America as the seventeenth least corrupt country in the world. While that puts it behind Denmark, New Zealand, Finland, Sweden, Norway, Switzerland, Singapore, Netherlands, Luxembourg, Canada, Australia, Germany, Iceland, the UK, Belgium and Japan, and tied with Barbados, Hong Kong and Ireland, it means that the U.S. is pretty good globally. That's in an era when corruption is less prevalent than it's ever been. The press release for the 2014 Corruption Perceptions Index hypothesized that America's low ranking vis-a-vis industrialized nations might be caused by "the murky nexus between money and politics in all levels of United States government. As the highly publicized trial of former Virginia governor Bob McDonnell showed earlier this year, there is a real risk that wealthy individuals are able to buy favors from elected officials. This risk is further compounded by the fact that state and local governments do not always have robust laws limiting acceptance of gifts. Moreover, the recent spate of corruption scandals in New York has also undermined the public’s confidence in the probity of state and local officials".
The Heritage Foundation, meanwhile, has ranked America lower than in the past as far as economic freedom and note that there is a perception of corruption. While they make some reasonable points about a culture that emerged under both Bush and Obama of patronage and corruption, Heritage's worldview is, well, bonkers. In any instance, even they put America as being very high as far as freedom from corruption.
State-level analyses have found that many states (including some quite red states like Mississippi, Louisiana, Tennessee, Alabama and Alaska, to rebut those who view corruption as a liberal problem) have serious issues with controlling government spending and dealing with corruption. There are also issues with Medicare fraud, military spending fraud, and so forth. However, even there, most of those schemes tend to have lower-level cohorts in the agency if they have anyone in the actual government departments as a co-conspirator.
Compare this to countries like Russia or Nigeria or Afghanistan. In countries like that, corruption goes all the way up and down the organizational hierarchy. There's no opportunity to get an independent regulatory agency to investigate. The media are powerless, and civil suits are simply impossible. A culture of intimidation silences and squelches those who want to confront the corrupt.
Heck, even in the past, we not only had much more serious election fraud in most major cities but also far more intimidation of voters and far more entrenched organized crime. RICO helped decapitate the Mob: they are nowhere near as powerful as they used to be.
So, yes, America is effectively an oligarchy now, given how little the opinions of the populace translate into policy. Yes, corporations have massive influence. Yes, inequalities have become entrenched. Yes, voting rights have been repressed in serious ways.
But we do not have rampant bribery, entrenched rent-seeking behavior, or totally meaningless elections.