Somebody: That's fine, but even if it's an indeterminate populism it seems unlikely that it does not have more of a coloration of right or left, on balance. Given that most of the major American myths are more supportive of a conservative populism, I think Wojtek has a point.
Here's a few examples: the myth of small government, politicians are all corrupt (but there is no analogous myth about businessmen), the free market, separation of powers, individual responsibility, upward mobility..
On the other hand, you'd struggle to come up with myths that are more compatible with left-wing politics. Do we have ingrained notions of solidarity, of resisting the bosses, or a tendency to blame capitalism for economic downturns or CEO's for unemployment? Certainly not. You can't blame this all on the force of ruling class ideology. In other places and other times the ruled classes have had their own countervailing ideologies. Outside of Wisconsin labor protests and maybe The Nation subscriber list, we do not.
Incidentally, I cannot help but note that your argument against American conservatism is to note that most people could potentially be rallied on behalf of Medicare and Social Security. But could they easily be roused to support single-payer or free college education? I doubt it. Americans are, on average, constitutionally conservative in an inchoate way, so their support for existing programs is not so surprising. I don't see them supporting an expansion of social democracy.