The federal testing program has been a tool akin to a ruler for 30 years, measuring students’ knowledge of basic subjects. Now, President Clinton and two leading presidential contenders want to turn it into a lever, giving states financial incentives to improve performance and close the gap between high and low achievers. While the proposals have bipartisan support, some longtime supporters of the National Assessment of Educational Progres question whether the federal government should tie a carrot to what traditionally has been a yardstick.