After five months of labor upheavals and national crisis, Poles have become accustomed to reshuffles among their leaders. But, even by the abnormal standards of the times, the return yesterday of Gen. Mieczyslaw Moczar to the ruling Communist Party Politburo is a remarkable political event.

In China it seems to happen fairly frequently. But in the recent history of the Soviet Bloc, it is exceedingly rare for a Communist politician to stage a comeback after having been eased out of office and discredited by his rivals. The reelection of Moczar, who made his first serious bid for power in 1967, reflects both his acute sense of tactics while he waited in political obscurity for almost a decade and the depth of Poland's present crisis.

Moczar's return to the innermost leadership circle was all the more sweet for him because it coincides with the disgrace of an old political opponent, former party chief Edward Gierek. The two-day plenum of the party Central Committee voted to expel Gierek from its ranks, accusing him of fostering an atmosphere of intrigue during his 10 years in power and creating a sham democracy.

A former interior minister, the 67-year-old Moczar spearheaded a campaign to investigate and purge Poland's Jews following the 1967 Arab-Israeli war. In his hands, the campaign developed into an attempt to topple the party leader of the time, Wladyslaw Gomulka, from power.

In March 1968, Moczar directed a police operation against Warsaw students demonstrating in support of the Czechsolovak reform movement unleashed by Alexander Dubcek. Propagandists were quick to label the student unrest as fermented by a "Zionist fifth column."

Moczar's bid for power failed largely because Bomulka's position was shored up as a result of the Soviet invasion of Czechoslovakia. It was clearly not the right time for a change of political leadership, particularly in view of the Kremlin's reservations about Moczar's brand of hard-line nationalism.

12 Years later, Moczar has attempted to soften his somewhat harsh public image. In recent speeches, he has gone out of his way to stress Poland's alliance with the Soviet Union and even praised the contribution of the Jewish community to Polish culture and its sufferings during World War II. He has projected himself as a politician with clean hands, untainted by the mistakes of the Gierek era, who can be relied upon to clear up the mess.

His elevation to the Politburo means that he is now one of the most powerful men in Poland. His many proteges in the leadership include the present party secretary. Stanislaw Kania, and the Politburo member in charge of economic reforms, Stefan Olszowski.

In certain circumstances, it would be possible to envisage Moczar as a rival to Kania. But the for moment, he appears to be going out of his way to bolster the party secretary's authority, which has been damaged over the last few weeks as a result of the continuing labor unrest.

Moczar's acknowledged political skill is illustrated by the way he managed to prepare his own comeback. Removed by Gierek from the Politburo in 1971, it looked as if his career was almost over when he was given the largely sinecure post of head of the governmental watchdog Supreme Control Commission.

It was a job which gave him little real power but a splendid opportunity to gather files full of compromising material on numerous party officials. Under Gierek, there was not much he could do with the information -- but suddenly it has become politically dynamite.

When the corruption became an important national issue following last summer's strikes, the power of Moczar increased. Delving into his files, he produced enough evidence of political misdeeds and financial wrongdoing to keep the courts busy for years.

The commission's powers have been substantially strengthened. Instead of being required, as in the past, to report to the government -- the very body it was meant to oversee -- it is now directly responsible to the Polish parliament.

Exploiting the corruption issue, Moczar also regained his old job as chairman of the politically influential veteran's association, which still has some 600,000 members.

Moczar's reputation as a nationalist stems from the fact that he was one of a group of Polish Communists who worked inside the country during the war. Later, there was a sharp clash between home-based Communists such as Moczar and Gomulka and those party officials who spent the war in Moscow and returned with the Red Army.

Many of the home-based Communists were accused of nationalism by the Soviet-backed faction and ruthlessly purged. Moczar survived the purge and was allowed to remain a Central, Committee member only in recognition of his wartime record as a partisan.

In a speech to the Central Committee just before his reelection to the Poliburo, Moczar took a balanced attitude towards Poland's present unrest. He called for an end to chaos and excitement, but talked in a generally conciliatory way about the new independent trade union movement Solidarity.