Bill Clinton isn't the only Democrat with a hefty lead in the polls. Surveys suggest the Democrats may be able to widen their 57-to-43 edge in the Senate by as many as six seats. In California, Representative Barbara Boxer and former San Francisco Mayor Dianne Feinstein seem headed for victory. So does Carol Moseley Braun in Illinois. Meantime, GOP incumbents in New York and Pennsylvania trail their challengers. And once-vulnerable incumbents Terry Sanford (D-N.C.) and Wyche Fowler Jr. (D-Ga.) seem secure.