HALLETTSVILLE, Tex. (AP) — The Southern Baptist leader of an antiabortion organization has sent 15,000 letters to clergy and laity, urging the de omination’s convention June 15-17 in Norfolk, Va., to adopt a resolution strongly opposing abortion. Rev. Robert Holbrook, coordinator of ‘‘Baptists for Life, Inc., says in his letter that ‘certain of the news media and abortion advocates’ have pictured past convention actions as aoe abortion, and the con vention should take a stand clearly against it.