Sure, there's more than one culture in the US. The Civil War alone proves that, and the ongoing dividing line between the red and blue states carries on that internal division. The poor and the nonvoters aren't represented, and many of them are disenchanted with their political system.

But I don't see how this supports your contention that culture is produced solely by the political system. American liberalism and conservatism each have complex cultural and historical causes. Liberalism, for examples, goes back to Protestant individualism and to ancient Greco-Roman humanism. Conservatism goes back to medieval and to ancient barbarity which amounts to crypto-social Darwinism. The American system of government is relatively modern, so it's more liberal than conservative in that it fosters the individual liberty to search for earthly happiness on the assumption that we're on our own in a godless world. Now, American liberalism ironically reestablishes dominance hierarchies, as in the dominance of the wealthy one percent, which then makes the political system more medieval than modern, as lobbyists capture the regulators, and so forth.

The upshot, though, is that we needn't explain all aspects of culture just in narrow political terms. We can turn to the history of cultural influences. Systems of government don't just fall from the sky. They're created or chosen and they develop based in part on history, and on the people's ethos and mythos.