Ask a stupid question, the old wheeze goes, and you get a stupid answer. So the answer should have come as no surprise when a Washington Post-ABC poll recently asked a scientifically selected sample of 1,007 adult Americans: ''Ultimately, do you think the United States and the Soviet Union should or should not agree to eliminate all nuclear weapons?''

A large majority of 68 percent said ''should.'' Only 29 percent said ''should not.'' A prudent 3 percent had ''no opinion.''

Given the awful complexity of the issue, and the banality of the question (when is ''ultimately''?), ''no opinion'' should have won hands down. But never mind; you can't fault the two-to-one majority in favor when not only Ronald Reagan but Mikhail Gorbachev is holding out the pie in the sky of a world without nuclear weapons.

Still less, when the top men are talking that way, can you blame the pollsters for asking people what they think of it? The problem, then, lies with the loose way the two world heavyweights talk -- and why.

Gorbachev's game is not hard to figure out. Leave aside whether he really thinks that such a state of grace could be verified, that nuclear technology could be disinvented, that lesser potential nuclear powers could be trusted to comply. The general proposition of ''de-nuclearization'' plays nicely to Soviet superiority in conventional forces and chemical weaponry in Europe. Gorbachev has nothing to lose.

But that is why Reagan has every reason not to talk in the same simplistic way. Why he does so has been puzzling me ever since he launched his Strategic Defense Initiative in 1983 and offered the prospect of a nuclear defense that would render offensive nuclear weapons ''impotent and obsolete.''

Does he believe it? Or does he recognize the pitfalls in the proposition, but see no point in letting details get in the way of dreams?

It's both. Or so I became increasingly persuaded, in the course of recent encounters with the president in the Oval Office together with three colleagues. It has to do, as well, with the catch-as-catch-can quality of communication with the Great Communicator: the shouted one-line exchanges over the roar of a helicopter; the scattered-shot questioning at rare press conferences; the infrequency, in short, of opportunities to push the follow-up questions that get you beyond the true beliefs to the complexities.

In our Oval Office exchange, there was an opportunity to follow up, in a way that illustrated the point. In an earlier interview with TV anchormen, Reagan had reconstructed the breakdown at Reykjavik in these words: ''We had come to an agreement on literally total nuclear disarmament'' when Gorbachev at the last moment brought up SDI, and ''that's when I came home.''

He was not then asked why, if ''literally total nuclear disarmament'' has been agreed to (as was not quite the case), SDI was any longer a problem. But no matter; the question that remained was put to him in our session: Was he not in danger of ''making the world safer for conventional war and putting the Europeans at some risk?''

It was like pushing the wrong key on a computer. The president launched into a long disquisition on the difference between conventional and nuclear war -- between the traditional ''rules of warfare'' and ''mutual assured destruction'' with ''missiles exploding all over'' the United States and the Soviet Union. He spoke movingly of how ''uncivilized'' it was for each side to hold the other's noncombatant men, women and children hostage to nuclear deterrence.

Fine, but was he still not worried about ''the conventional imbalance in Europe if you just did away with all nuclear weapons?'' Bingo! This time, the right key was pushed: ''That, of course, is most important,'' the president said; Gorbachev had even indicated to him that he was of the same mind. Before he would agree to eliminate the ''hundreds and hundreds'' of shorter-range battlefield nuclear weapons in Europe that ''equalize'' Soviet conventional military superiority, the president said, conventional ''parity'' would have to be achieved.

A few days later, the president hammered home the same point in a formal address carefully calculated to ease European anxieties. So, at one level, it cannot be said that Reagan is unaware of the enormously complicated strings attached to his vision of a nuclear-free world.

But at another, visceral level, the complexities come across as afterthoughts -- if at all. With Ronald Reagan, as with the public, it depends on how the question is put -- or which key of the computer is punched.