THE COMMUNIST Party Congress in Warsaw fulfilled its promise as a climax of the Polish revolution. The party said "me, too," to the principle the workers laid down last year: power flows from the bottom up. No other Communist Party has so legitimized itself. Poland still is a one-party state whose social order and foreign oreintation are beyond debate. But of the ruling Communist parties, Poland's alone now represents the will of its rank and file. This is a breakthrough whose consequences in Poland and elsewhere will be unfolding for years.

But if the congress "renewed" the party, it did not settle the larger question of where power now lies. The party ahs two million members. Solidarity has eight million and Rural Solidarity one or two million more. The Solidarity organizations have their own dynamics and agendas rooted in improving the life of the people. The party cannot command them in the old style. It can govern only by consulting in some new style yet to be worked out. Poland no lnger has a dictatorship of the proletariat. It is closer to a democracy of the proletariat, whatever that turns out to mean.

Many people say that, with the congress over, the party and Solidarity can tackle the economy together. The economy was in sharp decline before Solidarity bloomed last summer; that's why Solidarity bloomed. It has since been in sharper decline, with 15 percent of national income lost in one year. Optimists think it will take until 1985 for Poland to get back to the leve of 1978.

But it is too simple to say the party and Solidarity will tackle the economy together. One tendency in Poland calls for austerity and rform and a second for relief for the workers. More freedom has been gained. But who will now ask the workers to work harder, for less? Decentralization and a wider play for market forces are surely needed. And if these put workers, not to speak of bureaucrats, out of work? Everything could yet collapse.

The dog that didn't bark during the congress was in Moscow. The Kremlin apparently decided to let the Poles try it there way. Drossed fingers are essential, for Poland forever. But the fact of Soviet restraint is evident and welcome and has great political meaning. The Soviets are now giving breathing room and billions in aid to a development of immense potential strategic and ideological consequence on their very doorstep. Consider what the situation would be if the party congress, or the year, had produced a Soviet intervention. If Soviet anxiety about Poland is confined to growling, a burden has been lifted from East-West relations.

The Reagan administration is entitled to say it gave crucial support to Poland by removing an lingering Soviet doubt about whether the United States would reach harshly to an invasion. Now it must figure out how best it can help sustain the Polish experiment. The effort is bound to mean that Poland, though still socialist and still a Soviet ally, will move ever more inextricably into the Western banking and trading system from which Stalin forcibly removed Eastern Europe after World War II. The effort will also likely cast the United States as a silent partner of the Soviet Union in fortifying the hybrid that Poland seems to be becoming.

All this will require much thought and will entail considerable strain, but it is an eminently valuable enterprise. For it widens the freedom of the Polish people in a manners heretofore unimaginable. It greatly reduces, if it does not altogether end, the chances of Moscow's organizing an armed attack on Western Europe. And it sets up currents that will run through the Soviet bloc -- who knows how? -- for years to come.