BILL FACES A VETO
Senate, 64-36, approves bill to ban so-called 'partial-birth' abortions; abortion opponents fall three votes short of number needed to override Pres Clinton's expected veto; Sen Rick Santorum, who led fight for ban, comments; abortion opponents, who spent over $1 million in advertising recently in eight states to sway potentially swing senators, pick up only three votes; photo; graph (M)