Stating the obvious: Politicians know politics; that's their business. Business is not their business, and any discussion about American presidents and economics has to begin with this discouraging word: American politicians, with a very small number of exceptions, don't know anything about economics. In Washington, during the deficit debates for the past few weeks, politicians are guessing–as I think most economists and pundits are–and they seize on almost any deficit idea that sounds good at the time. It has been ever thus: A mainstream American conservative, Richard Nixon, blurts out that we are all Keynesians now, and a mainstream American……