In the face of near-certain defeat on health-care reform, the White House is planning a high-profile launch of President Clinton's national-service program, AmeriCorps. On Sept. 12, Clinton will lead 1,000 youths on the South Lawn in the service oath -- and 9,000 more will participate in satellite hookups at 16 sites nationwide. Also planned: MTV-style commercials plugging service and AmeriCorps merchandise from T shirts to backpacks. Why the Super Bowl-style hype? National service is one major campaign promise Clinton has kept, and aides complain he hasn't received enough credit.