For Mr. Trump, a presidential candidate whose appeal is predicated on an aura of toughness, personal achievement and perpetual success, the story of his brother Freddy, a handsome, gregarious and self-destructive figure who died as an alcoholic in 1981 at the age of 43, is bleak and seldom told.

In a telephone interview last week, Mr. Trump said he had learned by watching his brother how bad choices could drag down even those who seemed destined to rise. Seeing his brother suffering led him to avoid ever trying alcohol or cigarettes, he said.

In the upwardly mobile Trump family, Donald was the second and favorite son. Freddy was the disappointment, who lacked the killer instinct and drifted so far from his father's ambitions that his children were largely cut out of the patriarch's will.

Asked whether Freddy's experience in the family business, which friends described as miserable, contributed to the drinking that ultimately killed him, Mr. Trump said: "I hope not. I hope not."