SORTLY AFTER taking office, President Clinton began pushing for a stimulus program to end the country's recession. But according to the National Bureau of Economic Research, the recession was already over. It began in July 1990 and ended in March 1991. Since that time tile U.S. economy has expanded continuously. By the end of 1992, in fact, the economy was growing at an annual rate of 4.7 percent--almost twice the postwar average.