Senator John F. Kennedy tonight expressed keen disappointment on the defeat of the Anderson-Kennedy amendment to the medical care to the aged bill.
   "I am sure that the threat of a White House veto mentioned by every speaker opposing the amendment contributed to this defeat. In addition, it should be noted that only one Republican voted for this sound approach to the medical care for the aged program.
   "This vote demonstrates that if we're going to have effective legislation in this and other fields, we're going to have to have an administration that will provide leadership and a Congress that will act."