> There’s no monolithic “ruling class” that both major parties serve equally

There is a ruling class (capital), but it is not – nor is it asserted to be – monolithic. There are conflicts as well as common interests within it; the common interests are more consistently served by government bipartisanly because the dominant factions of each party answer to (different, but overlapping, subsets of) the capitalist class. The points on which the ruling class are also the points of genuine (rather than ginned up to build support on other issues) divisions between the parties.

The division in society is between the classes of people who live off their capital and people who live off their labour. These two groups of people have contradictory interests. People who own capital want to pay workers as little as possible and provide them with as few benefits as possible in order to maximize their profits. Meanwhile, the workers want to get compensated as much as possible, to get the best benefits they can, and to have time off. This is the class contradiction of the capitalist society.

In capitalist society, providing it develops under the most favorable conditions, we have a more or less complete democracy in the democratic republic. But this democracy is always hemmed in by the narrow limits set by capitalist exploitation and consequently always remains, in effect, a democracy for the minority, only for the propertied classes, only for the rich. Freedom in capitalist society always remains about the same as it was in the ancient Greek republics: freedom for the slave owners. Owing to the conditions of capitalist exploitation, the modern wage slaves are so crushed by want and poverty that “they cannot be bothered with democracy,” “cannot be bothered with politics”; in the ordinary, peaceful course of events, the majority of the population is debarred from participation in public and political life.

And this is precisely what the analysis of US government policy shows. US has a government by the rich and for the rich.