"Free Lech Walesa" read the protest signs still occasionally seen in Poland. But Solidarity's leader remains a prisoner, no less so for the fact that the physical conditions of his captivity are not severe--though he has just been moved far from Warsaw and his family--and that the misleading and pretty words "detained" and "interned" are often applied to him. Almost six months after martial law was imposed, he is still being held without charges and without visible prospects for release.

Mr. Walesa may be out of sight but, as those protest signs and much else indicate, he is not out of the minds of the people of Poland. The Communist authorities, whose pretensions to legitimacy he completely shredded, could not rule with him. They are finding now that they cannot rule without him. They cannot even do the one thing you would think they could do most easily--impose order. Street demonstrations and even a mini general strike have taken place since December. Underground Solidarity leaders grant interviews to the Western press. The authorities do not dare to take strict Soviet- type measures to ensure that these things do not happen again.

Most important, the authorities now know that Mr. Walesa's participation in a political dialogue with them is critical to their hopes of avoiding the sort of total breakdown that could yet precipitate a direct Soviet intervention, and of moving the nation toward some minimal measure of civil peace. The latter in turn is vital to the government's attempts to revive the still-desperate Polish economy.

Mr. Walesa, isolated but cool, is aware of his bargaining position. He is insisting that all others arrested in December be freed before he is freed--a tactic that has already helped return thousands of prisoners to their homes. He is also insisting that, in any talks that he joins with the government and the Catholic Church, his advisers take part with him--a demand meant to combat the government's effort to fragment Solidarity. All the evidence suggests that he remains a moderate, eager to carry his mass constituency into a workable compromise with the government. For the radicalization and cynicism that have unquestionably overtaken some parts of that constituency, the government, not Mr. Walesa, is entirely responsible.

The crucial element of Mr. Walesa's strength is his position in his country. But the support he receives in the West is very relevant and useful. The West has more or less given him its proxy: the economic sanctions imposed in January will not be lifted until the authorities allow him to resume a role consistent with his popular standing. Until now, at least, the bottom line of the West's collective policy, as ragged as it has sometimes appeared, has been: Free Lech Walesa.

In his current trip in Europe, President Reagan will be hearing various appeals to go back to business as usual with Poland. The basic argument is familiar: to continue withholding debt renegotiation, new credits and normal trade is to hurt the Polish people in the name of putting pressure on the Polish government, and to force Warsaw to shrink its ties with the West to the point that it will become irreversibly reliant on Moscow. Sanctions, in short, hurt the wrong party and they backfire: they don't work.

Conceivably there may come a time when that will turn out to be true, but that time has not yet arrived. The sanctions are working in the sense that, six months into martial law, they constitute one of the principal factors forcing the regime of Gen. Jaruzelski to continue to consider relaxing martial law. Significantly, though Solidarity was suspended on Dec. 13, it has yet to be banned. A realistic observer would have to say that it could still go either way. But as long as it could still go Solidarity's way, it would be a political and moral disaster for the West to take the lever of the sanctions out of Lech Walesa's hands.

Some Polish officials suggest it is unrealistic to expect a Communist regime to let Mr. Walesa back on the stage. It would be even more unrealistic for the regime to expect to run Poland its way without him.