By the way of illustrating the essential intractability of Poland's economic troubles, a distinguished Soviet affairs expert passes along a joke told him recently by a Polish friend:

"There are only two ways to solve our economic problems -- one is realistic, the other miraculous. The realistic was is that Christ will reappear with a swarm of angels over Poland. The miraculous way is that we Poles will solve it ourselves."

The Soviet expert is Vladimir Petrov, professor of Soviet studies at George Washington University. And the almost open-ended prospect of economic misery in Poland is one of a number of reasons that Petrov is one of the few close students of the Polish crisis who has consistently clung to the view that the Soviet Union won't try to crush the Polish reform movement by outright military invasion and occupation.

"They will intimidate, and use economic leverage and internal political pressure," he argues. "But there is no quick fix, except in terms of complete suppression. And there is no quick fix at all for the economy. The Soviets can't count on solving anything by sitting as military occupiers for any long period of time."

Petrov is weighing odds, balancing pros and cons. Like most of the experts in and out of government in Washington, he would concede that the Soviet leadership itself probably doesn't know what it may feel compelled to do at some point. And along with the rest, he also agrees that the period immediately ahead, right up to the July 14 meeting of the Polish Communist Party Congress, is going to be critical.

Under new and nearly revolutionary procedures, party members at the grass roots have chosen a Congress whose 2,000 or so members could conceivably elect a whole new Central Committee and ruling Politburo. "The entire ruling strata will be charged, down to the second echelon," says Dimitri Simes, a Soviet and Eastern European expert at the Johns Hopkins School of Advanced International Studies.

Simes is among those who lean toward the dark view that the Soviets may decide that the sheer unpredictability of the outcome is intolerable -- that they may move militarily to prevent the party congress from ever taking place. "The Soviet leaders are old and conservative, and they probably would perfer to procrastinate. But time is running out, and the temptation to intervene is great."

Dr. Vojtech Mastney, a colleague also specializing in Eastern European affairs, leans the other way, cautiously. A party congress was in the offing, he recalls, when the Soviets plunged into Czechoslovakia in 1968. But history, he maintains, need not necessarily repeat. He contends that the new freedoms won by Poland's Solidarity union and the party were established and increasingly accepted.

"The Russians can't know what's coming or how fast, if they don't move in," he says. "But if they do -- even if they can control the situation -- what next?" He thinks the Soviets must recognize that what would come next, apart from bloodshed, would be "virtual bankruptcy -- they would foreclose solutions more than they would solve anything."

That's Petrov's point, among others. His reasons for clinging hard to the conclusion that the Soviets, in the end, will not invade are worth noting if only because so many eminent authorities have loudly predicted Soviet military intervention so often that the other side of the argument gets lost. Shorn of a lot of their essential complexity, these are his principal points:

Poland is a nation of 35 million people "who hate the Russians." Occupation would "rally the Poles around a nationalist cause" and give the current movement an "anti-Soviet thrust" that does not now exist.

The Polish Military is at best a question mark. "From the colonels up, the military may be with the Soviets -- but soldiers fight wars." Meaning: a strong likelihood of continuing resistance, sabotage and unrest. Petrov reckons that the Soviets see a go-no-go decision "60 percent in terms of whether, in purely pragmatic terms, intervention would make matters better -- or worse."

The remaining 40 percent, he estimates, would turn on world opinion -- above all, Third World. "The repercussions of Poland would be much worse than Afghanistan, and last longer," he says.

Economic distress will weaken Solidarity, over time, or so the Soviets may think. "When people are standing in line, they have to blame somebody, and usually they blame the activists."

The Polish experience, being uniquely Polish, doesn't translate readily into trouble in the rest of the Eastern European bloc, which largely supports the Soviets but "doesn't want to see the Soviets intervene with military force."

"I'm willing to stick my neck out -- I've done it before," says Petrov. The last time was when he predicted the Soviets would not invade Afghanistan. "I was wrong, but so were the Soviets," he says. "They thought it would all blow over in two or three months."

And that, he concludes, is just one more reason to think "they won't want to make the same mistake again."