[LexBaird]

And very few if any people in position of power in US are recruited from the lower class. I assume that most of leading positions are occupied by the rich.

Class and wealth are not synonymous. There are many people in positions of power in the US that have risen from humble means. We do not have a class system here, as it is understood in Europe. There is no such thing as "rising above one's station". Power is the reward of the successful, and as long as success is available to all who strive enough, so is political opportunity.

[A. Pole]

There are many people in positions of power in the US that have risen from humble means. [...] Power is the reward of the successful

I am not sure what you mean by the success here. Are you saying that people who are poor have a good chance to be in position of power in USA without getting rich first? Or are you saying that even if the most of politicans are affluent still many of them "have risen from humble means" and their position is a just reward?

You can have upward mobility and oligarchy at the same time. Roman and Athenian republics are good examples of it.