FOR THE THIRD TIME in 28 years, an incumbent Democratic president faces real opposition to his renomination in the New Hampshire primary. In 1952 and again in 1968, the incumbent presidents, Harry Truman and Lyndon Johnson -- their policies and their performances -- were the central isues in their opponents' challenges. With Jimmy Carter, in 1979, it is different. New Hampshire, and the primaries and caucuses that follow -- will be up or down referendums more on his principal challenger, Edward Kennedy, than on the president himself. Strange as it might seem to some future political generation, Mr. Carter -- as he nears the completion of his third year in office -- can be accurately described as a political remainderman.

It is all quite baffling. An incumbent president, who, according to the polls, arouses little passion, inspires little confidence and even less antipathy. One the eve of his announcement for reelection, Mr. Carter remains, for many of his constituents, an undefined and in some ways remote public figure. In fact, he may simply have the misfortune of being president when the American people are not buying what he's selling.

In 1976, the non-Washington outsider looked pretty good to an electorate reeling from the disclosures of Watergate and the disappontments of Vietnam, especially an outsider who openly professed a total belief in the traditional values that were then pretty much on the defensive. Now it appears we want in a president those qualities that the incumbent -- any incumbent -- lacks. After Lyndon Johnson and Richard Nixon, American voters cherished openness in their chief executive and the prompt exit of all White House imperiousness. But we are a fickle lot (which Mr. Carter recognized when he reinstated "Hail to the Chief"). Now, according to virtually every survey of public opinion, we want forceful leadership in our president. A John Connally candidacy that would have been almost laughable during the national craving for political chastity, only one presidential election ago, is today an altogether serious enterprise.

The irony of Jimmy Carter's presidency is that he has never seemed quite so presidential as he has in these last terrible 30 days. He has demonstrated leadership in a uniquely difficult situation in stark contrast to the political amateur hour that describes much of his administration's relations with Congress, the Democratic Party and large parts of the political world. In spite of a tripling of the inflation rate, an unbalanced budget and two new Cabinet-level departments (instead of the promised cut from 1,900 to 200 federal agencies), the American jury is, to a large degree, still out on Jimmy Carter as he formally seeks reelection. Undoubtedly, in the months ahead his fellow citizens will come to know Mr. Carter better as he defends and explains his presidency and defines both himself and his vision of America for the next four years and beyond.