NOW THERE are two parties in the Soviet Union. One embraces people who believe that the discredited Communist Party can beremade into an instrument of national renewal. The other invites those who despair of modernizing the Communist Party and who believe a new pluralistic structure must be put in place to challenge it. The battered but still battling Mikhail Gorbachev heads the first party, which in fact he moved some distance in his reform direction at the congress in Moscow. But he did not move it nearly far enough for his erstwhile Politburo prote'ge', Boris Yeltsin, the elected president of the vast Russian republic, who by resigning from the Communist Party has now given the second party a formidable chief -- himself -- and a sharp profile as the opposition democratic left.

Not that the two parties are at this moment equal. The Gorbachev party, torn and unwieldy as it is, retains the powerful resources of national organization, material wealth and familiarity with command. The Yeltsin party is not even a party in the sense of structure, membership and program; it consists of a diffuse collection of bold individuals, reform factions and popular impulses. In the Soviet ferment, however, it is the party of promise. It could yet capture the immense discontent uncapped by the Gorbachev reforms. In so doing, it would no doubt stir a determined response from Mr. Gorbachev. To be effective will require an unaccustomed discipline from populist Boris Yeltsin, who pledges, for instance, economic progress without social pain. Still, it appears that the Soviet Union is in its laggard and erratic fashion following East Europe into multiparty politics.

Already it was clear that the Soviet Union had entered a phase of deep internal preoccupation. The Yeltsin phenomenon ensures it and is in that sense a gain. The West will have to graduate from a fascination with the clash of Kremlin personalities to serious study of the developing political scene. Any thought of ''helping'' or pinning inordinate hopes on one man or the other must yield to the normal official deference accorded to a sitting leader and the normal respect shown to a prospective replacement. One thing to keep an eye on is whether a Gorbachev government with a Yeltsin opposition can move more effectively to advance economic reform and put goods in the stores. Another is whether the Soviet Union can institutionalize pluralism, really make it work. On these matters hinges the Soviet fate.