ONE of the staples of political argument in the United States-more so than in other capitalist democracies-is the question of the desirability and effectiveness of direct government intervention into economic and social affairs. Most of the time, therefore, opinions follow the lines of regional, class, or other factional self-interest, and the two political parties look pragmatically for positions that will assure them the support of the majority they need to enjoy the benefits of office. Only occasionally does the rhetoric of the leadership slip into ideological gear. That is why it came as such a surprise that the Goldwater campaign of 1964 went so far as to attack a longstanding consensus policy like the Social Security system, and to attack it as a matter of principle.  When this happens, the debate often becomes acrimonious and facts merge with legend.