THE vice-president for two centuries has been the forgotten man of the American Constitution. He has been a kind of spare tire of government. Often he has been far out of things; Harry Truman, for example, had never heard of the A-bomb when he succeeded Franklin D. Roosevelt.
In the past 10 years, however, the stature of the vice-president has been subtly rising. He now has a home of his own on Massachusetts Avenue and traffic sweeps back and forth in front of the ''Vice-President's White House.'' The prestige of the job has grown a bit, too. One former vice-president, Walter Mondale, is ardently running for the presidency. On the Republican side many believe that Vice-President Bush will be a presidential candidate four years from now. The job offers greater national visibility than any other office save the presidency. It has evolved into a training ground on which potential presidents can learn their jobs in advance.
''The past two decades have witnessed major changes in the American vice-presidency,'' says political scientist Paul Charles Light of Brookings Institution. ''Despite all the jokes, the vice-presidency is now a very good job. Vice-presidents have finally joined that small group of White House advisers who work in the inner circle . . . given the presidential nominating system, vice-presidents must be considered the front-runners for future elections in their own right.''
In 1961 the vice-president had a staff of only a score; now he has about 70. He has his own stable source of funding and an annual budget of $2 million. Previously the vice-president never quite knew which of five different locations in government buildings was home; now his staff is consolidated in the Old Executive Office Building next to the White House.
The vice-president has a new seal and flag, too, introduced by Vice-President Nelson Rockefeller. The old seal showed an eagle at rest, now it is in flight clutching a businesslike clawful of arrows. The office of vice-president has been upgraded in more positive ways. Predecessors relied on presidential goodwill for information, but now a staff of experts in foreign and domestic affairs help sustain the vice-presidential advisory role.
Watergate gave a boost to the vice-president. In the twin crises of Spiro Agnew's resignation and of Richard Nixon's own departure, Gerald Ford was able to bargain. First he got more staff and independence for himself as vice-president. Then in turn he had more to offer to his own vice-president, Mr. Rockefeller. The same trend continued on the Democratic side when President Carter was elected and Fritz Mondale came to a more powerful vice-presidential office. ''It is safe to say that Mondale had more impact over administration policies than any vice-president in history,'' says Professor Light. Of course, Vice-President Mondale and President Carter did not always see eye to eye; they formed a united front on most subjects. But Mr. Mondale suffered his share of defeats like the 1979 midsummer cabinet firings.
In the strange American presidential-selection system there is no certainty in what direction any particular development will go. Nobody can be sure who is in charge, or whether the next deadlock will yield in time. These obstacles only make the potential of the incumbent vice-president greater. He can be exponent, campaigner, and crusader. Vice-presidents may also serve some of the duties of national party chairmen - the most visible surrogates of presidents. Will the recent trends continue? The present election may show.