A missing person provides the best clue to the joyless character of the primary campaign that has just ended. For the first time in decades no serious bid for the presidency has been made by a candidate from the traditional center.

The traditional center is easier to evoke by allusion than to describe with precision. It grew out of the New Deal coalition and emphasized government action to promote economic welfare and foil foreign bad guys. It was working-class in tone. It connected with the group for which, a dozen years ago, I coined the term Middle America. It was personified -- by Ben Wattenberg and Dick Scammon -- as the "housewife from Dayton."

Harry Truman, John Kennedy and Lyndon Johnson all commanded the traditional center. Sen Edmund Muskie was its candidate in 1972; Sen. Henry Jackson in 1976. Men from the traditional center still stand at the place where the working majorities of Congress come together -- witness House Speaker Tip O'Neill from Boston and Senate Majority Leader Robert Byrd from West Virginia.

But this year none of the presidential candidates even come close to fitting the mold. Republicans -- from Ronald Reagan through George Bush to John Anderson -- were far too conservative on economic policy. On the Democratic side, President Carter was both too conservative in economics, and too dovish in foreign policy. Sen. Kennedy had the right name, and the right approach to economic matters. But he was soft on the ayatollah and the Russians, and his style clashed with the symbols of family stability dear to the traditional center.

Reform politics is generally advanced as the explanation for the waning of the center in presidential campaigns. The rule changes of 1972 gave great weight to militant activists who "participate" in the political process on behalf of such relatively narrow interests as minority rights, ecological balance and abortion. Victory in the primaries has gone to candidates who put together bits and pieces of the groups working the fringe of national politics.

Other evidence, besides presidential politics, however, testifies to the breaking up of the center. A Gallup Poll of Oct. 5, 1979, found that only 12 percent of the country considered itself middle-of-the-road. Another 47 percent placed themselves on the right while 32 percent were on the left with 9 percent undecided.

Potentially powerful leaders with centrist characterics tend to be bored in office. For example, Gov. Hugh Carey of New York, or former governor Jack Gilligan of Ohio. Centrist institutions, for instance the Coalition for a Democratic Majority, take on the character of narrow and doctrinaire in-groups. Even the best centrist publications sound like broken records.

Events, no doubt, played a role in diminishing the center. The civil rights movement detached minorities and made them into specialized pressure groups. Vietnam withered the foreign policy credo of the centrist, and left many of them stranded on the beaches of the Cold War. Watergate shook everybody's confidence in the presidency around which the center used to group itself.

Television played a special part in fostering those developments. It bore hard on the old political virtues of service, loyalty and accommodation. It became a vehicle for promoting dramatic confrontations by loners supposedly given to straight talk.

But events didn't become salient by themselves. Nor did television acquire its characteristic bias in a vacuum. Willing hands sharpened the issues and imparted the message to the medium.

The willing hands represent what has been called a new class thrown up by the success the old system. The prosperity of the postwar years, and the mad rush to the elite universities, raised a crop of better-educated citizens with higher goals, finer sensitivities and more enlightened views.

This meritocracy has been the active force shaping modern American politics. It wrote the reforms that gave special weight to the primaries, and to the citizens most keen on "participatory democracy." It showed special concern -- that code word denoting moral superiority -- for civil rights, Vietnam and Watergate. It took over and now dominates television news.

The upshot is a new political system based on a new structure of society. Leaders advance themselves less by showing a capacity to get things done than by taking stances of moral or ideological purity. Government becomes far more difficult, and politics a sour business.