ON TYRANNY, 20 Lessons... -- the one-party state

Do we live in a one-party state, could or should be a question for us against the show and charade of our supposed two-party system at the federal level.   Perception and reality are not the same. 

Continuing with Timothy Synder's book, On Tyranny, is Lesson 3; here, he begins with this adaptation of his statement: 

We imagine our own righteous vigilance directed outward (against outside hostiles or threats). We see ourselves as a city on a hill..looking from threats from abroad. But the sense of the saying was entirely different (referring to the saying "eternal vigilance is the price of liberty"): that our nature is such that [liberty] must be defended from [internal threats; those who aim to take advantage of liberty] exploit freedom to bring about its end. 

He sites historical evidence, the outcome of a one-party system in and beyond Nazi Germany.  Once again, he responds, 

Certainly the early Americans who spoke of "eternal vigilance" would have thought so (referring to an internal threat). The logic of the system they devised was to mitigate the consequences (of a threat to liberty).

What are the risks, threats? 

We certainly face, as did the ancient Greeks, the problem of an oligarchy--even more threatening as globalization increases differences in wealth. The odd American idea that giving money to political campaigns is "free speech" means that the rich have far more speech (or power) [than the rest of us].   We believe that we have checks and balances...

But do we in an oligarchy...governed by a relative few of non-elected, wealthy?
Are there real checks and balances?