The threat to public schools arises from their defects, not their accomplishments. In small, closely knit communities where public schools, particularly elementary schools, are now reasonably satisfactory, not even the most comprehensive voucher plan would have much effect. The public schools would remain dominant, perhaps somewhat improved by the threat of potential competition. But elsewhere, and particularly in the urban slums where the public schools are doing such a poor job, most parents would undoubtedly try to send their children to nonpublic schools.