Francis B. Francois the senior member of the Prime George's County Council, has announced that he will not challenge County Executive Winfield M. Kelly Jr. in the 1978 Democratic primary.

By taking nearly 10 pages to explain why he will not run. Francois may have set a new standard in the increasingly popular political art of non-candidacy announcements.

According to Francois, 43, a Bowie resident who has been called "the philosopher-historian" of the council, he had the support, experience adn desire ti run for the county's highest office next year. He said he even had enough money.

But, he explained Monday, "I can be most effective for the people of Prince George's County during the imediate future by not spending 1978 in what would clearly be a hard-fought campaign for County Executive, and instead applying whatever God given talents and abilities I have to the County Council and the related governmental matters in which I am now engaged."

This was interpreted by many of Francois' colleagues in Upper Marlboro to mean that he had concluded that he could not defeat Kelly, who already has a campaign treasury approaching $70,000 and will be at the head of the organization ticket.

Kelly's supporters, who have treated Francois with equal measures of disdain and respect since Kelly took office in 1975, reacted happily to the announcement. "This means there isn't a serious Democratic contender on the horizon, if God is in heaven," said one.

Francois, a liberal with close political ties to Rep. Gladys Noon Spellman (D-Prime George's), has been labeled a maverick because of his frequent disagreements with Kelly and other council members.

Despite his trouble in Upper Marlboro, Francois had developed a role as Prince George's ambassador to the outside world. He travels more often than his fellow council members, serving on a variety of boards and commissions from the Metropolitan Washington Council of Governments to the National Association of Counties.

In his 10-page statement, Francois indicated that he was comfortable with his role and did not find it worthwhile to jeopardize it by chalenging the organization. He was critical of Kelly's "new quality" campaign, saying it does not deal with problems of the poor and elderly. But he praised Kelly's administration for being honest, promoting the county economy and "moving ahead" in the field ofhuman rights.