The smile is the final guarantee. With it, Vernon Jordan can't be missed.

It kind of eases up from deep inside him, lingers momentarily, then bursts into a flash of perfect, gleaming white teeth that could make the toothpaste industry green with envy.

It rarely fails to leave the recipient rosily beaming back - except in the case of President Jimmy Carter.

As executive director of the National Urban League for the last four years, Jordan has used that smile - along with a Southern, country drawl, in dispatching dissension from the League's ranks, garnering corporate dollars for his organization. Most recently he used it on U.N. Ambassador Andrew Young, after telling Young's boss that the Carter administration isn't living up to the support blacks have given it.

"I think the President got the message," Jordan said as he made his way, smiling and greeting league members, through the halls of the Washington Hilton Hotel where the league held its 64th annual convention this week. "He may not like it, but I expect the administration will be responsive in the days and months ahead."

Jordan was on his way to an interview session with reporters. He towers over people, tall, black and imposingly handsome. Hellos greeted him from all sides, as did congratulations for "telling the administration the way it is," as one man puts it.

"It needed doing," another man said to him before Jordan disappeared into a room that had been set up for the interviews. During the next hour, he repeated his criticism of the Carter administration, his hopes for the future, and his views on civil rights organizations in the 1970s. The smile had gone as Jordan turned serious.

In earlier times, Jordan has been taciturn, even glum, playing his cards close to his chest. But from the start of the convention, Jordan was "up."

Then President Carter took him behind closed doors and rebuked him for his criticism of the administration, and some conventiongoers noticed a change. A chastened Jordan, a man whose face evidenced an occasional flicker of uncertainty. And they wondered why, since Jordan's expressed disappointment, they said, was widely shared in the black community.

At 41, Jordan is viewed by many as the senior civil rights spokesman on the national scene. A product of th civil rights movement, part of the Atlanta Mafia of black lealders that includes Andrew Young and Atlanta Mayor Maynard Jackson, Jordan spans the gap between the earlier black leadership that was limited to getting civil rights on the legislative books, and those who now are dedicated to seeing the law carried out.

"Black leadership permeates almost every aspect of American life now," Jordan said while riding to the State Department where his wife, Shirley, was co-hosting a lunch with the wife of Secretary of State Cyrus Vance. "We've undergone a fantastic democratization process. Besides the growth of the traditional black middle-class leadership, there are people emerging from community leadership." He went on to cite the number of black members of Congress and blacks in the administration.

"But the whole of black leadership can't go to work for the government," he said, explaining that he turned down a Carter offer for a Cabinet post "because I think there is a need for an outsider - a constructive, positive leadership role That way the nation will have a better sense about black attitudes and plights.

"That's what encouraged me about Carter's visit.He coming, his offering a parttership, his willingness to confront a constituency that had expressed some disappointment with him."

Jordan grew up in Atlanta, the middle son of a government worker father and a caterer mother. "Both my parents were hard workers, and my mother had a good, solid business," he said about his background. For almost 10 years of life, the family lived in University Homes for Black People, among the first public housing units in the country. Then they moved to what Jordan calls "the little while dream house with green shutters that everyone aspires to."

Though segregated, Atlanta was an intellectual mecca for blacks, and Jordan says he grew up seeing blacks, college presidents, lawyers, doctors, YMCA administrators and the "great civil rights lawyer, Col. A. T. Walden." One of the most special days for young Jordan was Emancipation Proclamation day, every Jan. 1, when his entire family went to hear people like Adam Clayton Powell or Thurgood Marshall speak.

"One of the greatest joys of my life was in giving that speech in 1965 when Walden was an old man in braces," Jordan recalled. "When I finished he said, 'Vernon, you hit a home run." I was so proud of this compliment from a man I considered a great warrier."

After college at De Pauw University, and law school at Howard, where he met his wife, Jordan returned to Atlanta, where he planned on being a civil rights lawyer. As a law clerk, he escorted Charlayne Hunter and Hamilton Holmes during the desegregation of the University of Georgia.

"He did everything then," recalls Hunter. "He ran for the sodas, the sandwiches, the blankets, whatever was needed. And he unearthed the crucial information that led to our entering." She recalls that he had to search the school's records to find a case similar to hers since the university contended that as a transfer student from an institution on the semester system, it was incapable of bringing her into its quarter system. "He's probably one of the most dynamic men in America," she says.

Jordan went from lawyering in Arkansas and Georgia to working with the NAACP and the National Negro College Fund. After Whitney Young's death, he was selected as executive director of the league. In his rise, though, Jordan has gained his share of critics who accuse him of not doing enough in his position of leadership.

"He was the only leader on the scene during the Nixon administration, and while he said a few things, the field was entirely his. He could have done more," says one critic.

The National Urban Coalition's Carl Holman argues, however, that Jordan, "like many strong-willed people, as he made it to the top, hasn't always treated certain people kindly or lightly. He has a pretty solid sense of his own worth, which is disturbing to some folks who might consider him arrogant. But he has a sense of who he is and what he wants and is not chary about expressing himself. He's an extremely able person and as a human being, he's a very witty fellow, down-home.

"In the South, he's what you call a young man an old head, because he's always been a good deal more thoughtful than his years. He's ambitious in the best sense of the word and a person who does get indignant about situations. Vernon is a more complicated person than he would sometimes appear to be when he is smiling or gladhanding."

Holman also says that Jordan is "probably closer to wealthy white people in this country than most other blacks. It enables him to be one of the most effective fundraisers around, and in bringing in resources to the league, they are available for the organization's work."

Jordan serves on the boards of Xerox, J.C. Penney, Celanese Corp., and Bankers Trust Co. He is also on the boards of the Rockefeller Foundation, the John Hay Whitney Foundation and the Taconic Foundation. He moves as easily in corporate circles as he does among league members and community political people.

Yet he maintains that he isn't interested in politics, at least not in an elected position, though at one time he considered running for mayor of Atlanta or for Georgia's 5th Congressional District, a seat that Andrew Young was the first black to fill.

"I sort of buried the idea then and there," recalls Jordan. "I do longer have any interest in elected politics. I like what I'm doing now. I like people and the sheer excitement of working with people on the local level. I like talking to Presidents, senators, chief executive officers and foundation heads, and because I spend a lot of time out in the local communities, I can tell them what people are thinking.

"I see myself as one whose job it is to tell a story, to tell it accurately, and to influence and bring about some change."

Jordan, his wife and daughter, Vicky, live in Westchester County in New York. They selected the suburbs because it was easier of his daughter to help his wife, who has suffered for the last 12 years from multiple sclerosis, Vicky, 17, is going to college this year, however, and the family will move into the city.

Friends say that Mrs. Jordan was reluctant to move, but Jordan convinced her by winning and dining her one night, taking her by the apartment, and showing her the moon over Central Park.

But Jordan does not talk so intimately about himself. "I think it's in bad taste. You just be yourself."

Is this part of his arrogance that "Arrogant?" he said, starting to smile. "That's what Andy Young said last night."

He was gone, gladhanding down the hall, with a smile to meet Attorney General Griffin Bell.