Can oil and democracy mix?*** WASHINGTON -- Call it the curse of crude. The 11 countries that comprise the OPEC cartel amassed nearly $180 billion in oil revenues last year. None is a thriving democracy. Coincidence? Oil wealth hinders development of a tax-paying middle class, the very segment of society most likely to agitate for a voice in government, political economists say. Bountiful crude reserves also discourage the kind of diversification needed for a successful capitalistic economy.***