The defense attorney for accused assassin Dan White told jurors today in opening arguments that his client was mentally ill from early manhood and "was unable to cope" with the intense pressures of San Francisco's sometimes turbulent political life after his election as supervisor.

White is charged with first degree murders in the deaths of San Francisco Mayor George Moscone and city Supervisor Harvey Milk on Nov. 27, 1978.

He has been charged under the "special circumstances" aspect of a new state law that provides for the dealth penalty when a public official has been killed or a mutiple death is involved.

White is pleading not guilty despite the fact that his attorney acknowledged today that he shot Moscone and Milk. White has the option, under California law, of pleading not guilty by reason of mental "diminished capacity." While not an insanity plea, it suggests that the defendant was mentally incapable of the full and malicious intent necessary for a first degree murder conviction.

While White has not yet taken advantage of the "diminished capacity" plea, were he to do so he could hope for a manslaughter conviction, which carries a maximum penalty of four years in prison on each count.

The former city supervisor "was fair, perhaps too fair for politics in San Francisco," said attorney Douglas Schmidt in his opening statement, which pointed up White's stress. "He believed a man's word was his bond."

Schmidt called White "idealistic," "a believer in traditional American values of home and family," "almost rigidly moral," who "saw the city as a deteriorating place for decent people and families to live."

Schmidt plans to call as many as 40 of the city's politicians and political activitists as witnesses. He pointedly said that "a man like Dan White could not have been elected" under the old citywide elections system. San Francisco switched to a district election of supervisors two years ago, in the election that brought White to office.

Prosecutor Thomas Norman's opening statement was a brief outline of major events leading up to the double murders. Norman avoided any mention of White's possible motive except to note that "subsequent to tending his resignation, he had feelings . . . that he wanted his job back."

White quit his job as supervisor on Nov. 10, citing financial difficulties, then tried to withdraw his resignation four days later. Mayor Moscone initially promised reappointment, but political pressures apparently persuaded him to appoint someone more philosophically sympathetic than the conservative White.

The prosecution must show that "special circumstances" existed when the murders took place. Norman told the jury both victims were "duly elected" officials who were killed to prevent them from carrying out their duties.

The prosecution's first witness was coroner Boyd D. Stephens, whose clinical testimony, complete with black and white photographs of Milk's corpse, outlined the slaying methods. Norman seemed to be attempting to develop evidence to show that the murders were professionally executed.

Moscone was shot twice in the body and, after falling to the floor, twice in the head. Milk was shot three times in the body and twice in the base of the skull.

Norman said the shots to the skull were "not unlike coup de grace shots."