Read other letters about this article
It wasn't the Republicans or the Democrats. It was the Republicans and the Democrats. Both pushed deregulation for years.
Both manipulated the tax code to favor "investment" income (long term cap gains and dividends get preferential tax treatment over grandma's passbook savings account interest), pushed home ownership as some sort of panacea for American families, gave in to corporate lobbying to repeal long established regulatory frameworks and just generally bought into the whole infallibility of markets line of BS. So in that regard, Posner is right. It was bipartisan. (And it still is.)