The candidate who emerges from tomorrow’s hard-fought primary with the Democratic nomination for Manhattan district attorney will be only the third person elected in 68 years to head one of the largest and most influential prosecutor’s offices in the country.

Robert M. Morgenthau, 90, who is retiring at the end of the year, has been elected to the job nine times. His predecessor, Frank S. Hogan, was first elected in 1941; he resigned on Dec. 26, 1973, after suffering a stroke and died of cancer a few months later.