The debate cracked the presidential riddle for at least one previously undecided citizen -- me.

Until Tuesday evening I had been leaning toward John Anderson, on grounds that his ideas were appealing and that a nod for him would send to the parties -- all right, especially to the Democratic Party -- a warning to provide a better choice the next time around.

It makes a difference that I was watching the debate with a friend visiting from Poland. Earlier, we had been discussing the fragile situation there and the idea had emerged that what imparts a gratuitous edge of peril to it is the lack of a live tradition of political accommodation. The Communist Party, being the lone party, is unaccustomed to the competition of the new union movement, and the union leadership has no experience of this sort itself. That very afternoon a confrontation had risen over a peremptory union demand to the prime minister to present himself, to thrash out a particularly difficult matter, on the workers' turf in Gdansk.

My polish friend and I agreed that a style of political communication between the government and the union movement must be devised so that the great and all too predictable shocks still to come in Poland -- a coal shortage affecting the heating of homes in the middle of winter, for instance -- might have a chance to be absorbed.

It was with this under our belt that the moment arrived for Ronald Reagan's comments on the SALT II treaty. Obviously he had it in mind to answer the charge that he is insufficiently respectful of the contribution that arms control could make to American security, and so he said he would sit down to negotiate with the Soviets for as long as it took "to have not only legitimate arms limitations but to have a reduction of these nuclear weapons to the point that neither one of us represents a threat to the other." He repeated the last phrase a few minutes later.

I do not want to overburden a set of words that Reagan could hardly have meant to be taken literally. But there was a little click in my mind at this evocation of a perfect nuclear risk-free world. It did not ring true. Reagan, I thought, was sniffing at "legitimate arms limitations" and was holding out as a feasible goal of arms control what he knows to be unattainable. If his purpose was to convey a willingness to negotiate, the effect on me was just the opposite.

The trouble with Reagan, I thought, is not that he is trigger-happy or lacks Oval Office experience or that he believes in strength and toughness. It is that he has yet to aknowledge, by word or reflex, a parallel need to develop a method of political communication with the Soviet Union. It is not simply that the specific features of SALT II might be lost: their value, we know, is nothing if not highly debatable. It is that the United States and the Soviet Union are locked into a totally engaged, totally deadly nuclear embrace that compels them to work a way to talk to each other so that they will not someday, in a shock, without meaning to, slide into catastrophe.

Like it or not, SALT is the principal -- the only -- medium of sustained dialogue between Moscow and Washington on the central question of the relation of the two countries' capacities to destroy each other and the world. A SALT gambit that could easily have the effect of breaking off this dialogue would replace uncertainty with a void. It would be especially dangerous to risk the thread of this dialogue at a time when Soviet-American relations are raw and possibly getting rawer, and when a change in Kremlin leadership is overdue.

Reagan's attitude appears to be simply, and simplistically, that American strength must be brought to bear and that sooner or later the Soviet Union will bend. Power speaks, he says, as though people did not also have to speak. The idea that a readiness to stand up for our legitimate interests need not be inconsistent with a readiness to respect legitimate Soviet interests seems foreign to him. He demands a "margin of safety," seemingly unaware that no great power will accept the flip side, a "margin of risk."

I felt that Carter, for all his failings, does understand this overarching 20th century reality and that he can do what Anderson cannot do -- defeat Reagan.

Such, at any rate, is how I concluded the real debate, the one all of us have been taking part in, the one with ourselves.