Boris Yeltsin, the new president of the Russian republic, has a talent for bodily punctuation. Just after predicting imminent civil war or the collapse of the Gorbachev government, he will arch his brow and twist the thick meat of his face into a smirk. Then, as his delirious audience reacts, he leans back in his chair like a king finishing a huge meal.

Yeltsin's style, a theatrical blend of the Communist Party apparatchik and the burlesque populist, sends shivers down the spines of some of his supporters in the radical intelligentsia. The late Andrei Sakharov shared committee chairmanships with Yeltsin. They helped start the inter-regional group of radical deputies. But those closest to Sakharov say that he was too suspicious of Yeltsin to vote for him in the March, 1989 legislative elections. Even now many of Yeltsin's public supporters wonder privately if they can trust him to put aside his ongoing opera of personal vengeance for the public good.

As president of Russia, Yeltsin has finally completed the sweet revenge he has been seeking ever since his fall from Communist Party grace in 1987. His domain now stretches from Leningrad to Vladivostok. For all of his vanities, Yeltsin now has the potential and the popular support to hasten two critical developments in Soviet public life: the further radicalization of Mikhail Gorbachev and the encouragement of a moderate, constructive form of Russian nationalism.

Nationalist movements in the Baltic republics and elsewhere in the Soviet "periphery" have heroic connotations in the West; they are movements of liberation, a challenge to an empire system characterized by the forcible destruction of community, religions and languages. Russian nationalism, however, is something else in our understanding -- a dangerous trend that could lead to some sort of clerical, authoritarian state dominated by antisemitic, anti-Western monarchists. Or something.

There are good reasons for the fear. It's hard to decide which is more frightening, the half-cracked young Pamyat members with their black T-shirts and "Kill the Yids" placards or the world-class intellectuals, such as the mathematician Igor Shafarevich, who have adopted the Stalinist habit of finding "Russophobic" enemies at every turn and then presenting their "scholarly findings" to the readers of Our Contemporary and Young Guard.

But the lunatic right is still mercifully tiny, and its leaders lose nearly every election race they enter. The West needs to understand that Russian nationalism is a far broader banner, one that includes ecologists who crusade for the salvation of Lake Baikal; the cultural historians who argue for the independence of the Russian Orthodox Church and a revival of Russian classical studies; and the politicians who campaign for the economic development of a land with endless resources -- and endless poverty.

Yeltsin, for the most part, emphasized these liberal nationalist positions in his campaign for the republican presidency. Like Gorbachev, he favors a new treaty of the Soviet Union that would provide unprecedented political and economic autonomy for Russia and the other republics.

But so far both Yeltsin and Gorbachev have been reluctant to alienate too many of the nationalists whose politics are rooted in the soil of resentment. At a public meeting with Muscovites recently, Yeltsin was uncharacteristically evasive when asked about antisemitism. Gorbachev, for his part, shocked people earlier this year when he appointed to his new presidential council Valentin Rasputin, a Siberian novelist and environmentalist who has flirted with Pamyat and blamed the Jews for both the Bolshevik Revolution and the purges that came after. Yeltsin could lead the way here with a series of clear gestures on nationalism.

Yeltsin could also find opportunity in his rivalry with Gorbachev. The two men began as the closest of colleagues in the Politburo. But even now, as they trade insults and accusations at every opportunity, their differences are more personal than political. Their battle has less to do with policy than with interoffice politics played out on the grand, if rickety, stage of the Communist Party. With his personal popularity, Yeltsin could help Gorbachev move faster on economic reform and build a better relationship with his natural allies on the left. But he will have to drop the ad hominem attacks before Gorbachev, a man of thin skin, will listen.

When Sakharov died last December, the radical wing of Soviet politics lost its singular voice, a man of a transcendent sense of judgment. Now the Soviet Union is left with two prominent leaders, both mortal, both prone to error. If Mikhail Gorbachev and Boris Yeltsin continue to play out their personal battle, one of them might win, but a poor country will be all the poorer.

The writer is The Post's Moscow correspondent.