With 663,000 more jobs disappearing from the American economy in March, the government's response to the downturn is being put to a strenuous test. Three months into the year, the unemployment rate has already soared to 8.5%, from 7.6%, the highest level in more than a quarter-century. The severity and breadth of the job losses in March prompted economists to conclude that an agonizing plunge in employment prospects was still unfolding.