For Minnesota, A Late Debate In a New Race
Former Vice Pres Walter Mondale, who replaced late Democratic Sen Paul Wellstone in Minnesota's US Senate race week ago, takes part in televised election-eve debate with Republican challenger Norm Coleman in St Paul; both stake out distinct positions on major issues in last-minute showdown that pits iconic older statesman against can-do up-and-comer; Mondale portrays opponent as right-wing tool of White House who cannot be trusted; Coleman recites record as jobs-builder and says Mondale's is precisely kind of tone he hopes to change in Washington; debate described; photos (M)