THERE IS a powerful inclination to want what you can't have in American political life, an instinct in both parties to seem always to favor the candidate who hasn't yet announced or -- more tempting yet -- who has announced that he's out of it. But we think that something more than this habit of disparaging acknowledged contenders and looking yearningly elsewhere was involved in both the eagerness many Democrats expressed for a Sam Nunn candidacy for president and in their disappointment that Sen. Nunn is not running.

Sen. Nunn gave as his reasons for resisting the pressure of friends and colleagues that he did not fancy the impact of the enterprise on either his family or his Senate responsibilities. These are understandable concerns. The assault on the family of a candidate can be merciless -- this is part of the price of the office, it seems. As for his Senate duties, Mr. Nunn only recently became chairman of the Senate Armed Services Committee, a position he had long trained for and aspired to and which is at the present moment the fulcrum of much crucial business. But the Georgia Democrat's statement yesterday that he will not be a candidate in 1988 ended, at least for now, something more than one man's presidential ambitions. It left the Democrats without a strong conservative-centrist contender in the race and left unanswered the question of whether the party can in fact even field such a candidate any more.

That's important because there are important strengths in this wing of the party. It ignores these at its peril, not just because by doing so in the past it has driven away voters and yielded the crucial center at the polls, but because by ruling out this part of its tradition for a time -- declaring it somehow illegitimate -- the party weakened itself intellectually and in terms of the policies it could espouse. Sen. Nunn's candidacy would have tested whether this period had passed.

For make no mistake: if Sam Nunn had chosen to run he would have encountered very strong opposition from many in the liberal wing of the party who, though admiring some aspects of his record and conceding his political stature and potential, would have considered him not just a man who had not in the past always voted in a way they favored or could understand, but a man on the other side of a line -- a line dividing candidates they could accept and support from those they could not. The Democrats are generally very forgiving of others in their party with whom they disagree on certain issues; the trouble is they tend only to forgive them after they have first chopped them up, declined to help them in a campaign and then lived to regret what they got instead, at which (useless) point they will summon the finished one up to the platform for a little hugging and cheering and empty auld lang syne.

There are votes Sen. Nunn has cast and positions he has taken which we disapprove of, and the good Lord knows we could say the same of all the candidates running. Anyone could. The question, at least where the Democrats are concerned, is whether it is any longer possible for a candidate occupying the constructive part of the political spectrum that Sen. Nunn occupies to be nominated for president. There's no candidate in the race now testing that proposition. There is also, in our view, no candidate in the race who has anything approaching Sam Nunn's credibility on foreign policy and national security issues. We are sorry he chose as he did.