In the marbled splendor of the Dirksen Senate Office Building, a short, red-bearded rabbi in a black suit and hat ran into a newly elected Jewish senator, tall and silver-haired.

"Are you here on Soviet Jewry?" the senator asked.

"No," the rabbi replied. "I am here on the whole world."

The rabbi, Abraham Shemtov, believes that there is no issue that is exclusively Jewish, least of all what he calls "the human's basic right to choose where to live." On the other hand, he says, both index fingers thrust didactically upright, "there is no such thing as non-Jewish." Then he concludes, fist clenched and thumb poised sideways: "A Jew has the responsibility--and in Washington, the opportunity!--to influence the world with a message that was handed down from Sinai, where Moses received the Ten Commandments. Everything is a Jew's issue!"

Shemtov, 45, was born in Moscow and came to the United States in 1953. For the past eight years, he has been a Washington lobbyist, but he prefers to be called "a messenger."

"A lobbyist is someone who wants to have someone do something for him," he says. "I am here to help people to do something for themselves." His aim is "to blend high moral values and low mundane publicity.

"Washington is preoccupied with the means," Shemtov says. "My job is to remind people of the end--the overall purpose of life."

The Lubavitch movement, which Shemtov represents in Washington, is led by Rabbi Menachem Schneerson of Brooklyn. Schneerson, 80, a tradtional Jewish sage and electrical engineer, is the direct descendant of the movement's founder, who lived in the Russian village of Lubavitch about 200 years ago.

With presidential blessings, Shemtov has organized Lubavitch projects every year since 1979. Typical is the 30- foot-high, nine-branched "national menorah" erected at Lafayette Square to celebrate Hanukkah. "Our law says that the menorah has to be in a conspicuous, prestigious place," he says, "so we put it as close to the White House as possible. That's where it belongs."

Another cause that takes up much of Shemtov's time is an annual congressional declaration of the "need for the nation to set aside a day devoted to the importance of education to the lives of its citizens and to the general well-being of the nation." Last year, that day was called "A National Day of Reflection," and the hope is that the day will become as nationally recognized as Mother's Day.

The Lubavitch movement, a blend of East European mysticism and American pragmatism, claims an estimated 600,000 followers and sympathizers around the world. In Washington, Shemtov's assignment is to pursue the Lubavitch goal "to contact and activate a commitment to a higher purpose in life--a purpose higher than one's self."

He meets with the president three or four times a year and takes part in White House conferences on families, the aged and the youth. He calls on senators, cabinet members and columnists.

"I talk to people who were not brought up the way I was brought up," he says, and he points his thumb at his velvet skullcap and his sidelocks draped around his ears. "But I am completely comfortable with them. It comes from being completely comfortable about my faith."

He is such a strict observer of the Jewish dietary laws that there isn't a Washington restaurant where he would have a cup of coffee. When he does join people for a meal, he brings his own, and he turns the event into a traditional Jewish study session, discussing a passage from the Five Books of Moses or from the commentaries on the Jewish law known as the Talmud. A typical guest list includes congressional aides, lawyers, an assistant secretary of commerce, and perhaps a White House aide-- up to 20 people.

The session takes place in the conference room of a law office. Shemtov, always in a hurry, shakes hands with all his guests--except for the women, to whom he explains that "I don't take what doesn't belong to me." Or he cites the Jewish law against a man touching women other than his wife, mother or daughter.

Then he opens his sleek black attach,e case and takes out a tray of lox, whitefish and sable--a selection of superior smoked fish carefully wrapped in plastic and then in aluminum foil. There are also plastic cups filled with potato salad and cream cheese, and a loaf of rye bread. The package is from a kosher delicatessen in Philadelphia, where Shemtov has a congregation, and lives with his wife and their six children. He commutes to Washington once a week.

The study session lunches used to be once a month. But, he says, his schedule has become so busy that in the past two years he has had time to organize only a few such lunches.

Shemtov says he is absolutely convinced that there is a great demand for the message he carries. "Believe me," he says, "the problem is not with the message or with the people we are trying to reach. If the message doesn't get across, I blame myself.

"We Jews are often missing the boat. The world is not deaf or stale. No, the world is eager to hear what we have to say. And I am fortunate to work for a man who has a message. He is the conscience of the world."

Asked if he is working for rebuilding the Temple in Jerusalem--which pious Jews believe will coincide with the coming of the Messiah-- Shemtov smiles. "We have to build a better world," he replies, with an impish twinkle in his eyes. "God will take care of the Temple."