SINCE Boris Yeltsin took power in Moscow in 1991, the United States has relied on him to guide Russia out of communism and into a bright future of democracy, free enterprise, and benign relations with the newly independent states that form Russia's ''near abroad.''
Four years later, Mr. Yeltsin has become an increasingly shaky reed to lean on. With his popularity gone, his health in question, and Russian national elections looming, the Clinton administration must contemplate a future without him. It is not a welcome assignment.
''The administration is forced to look very hard beyond Yeltsin but to do it in a way that's not offensive to Yeltsin,'' says Leon Aron, a resident scholar at the American Enterprise Institute, in Washington. ''That's a difficult task.''
Hard-drinking and unpredictable, Yeltsin has had a complex relationship with Washington but has been valued as a counterweight to both resurgent communists and ultranationalists. Anxious US officials, concerned about Russian stability and European security, worry about both alternatives as Russia approaches what could be a historic transition.
As they look ahead, US policy- makers assess two possible futures for Russia, which will be determined by the outcome of the Russian elections - for parliament in December and for president in June.
In one scenario, considered less likely in light of his recent health problems, Yeltsin stands for reelection and wins. In this case, US officials say, there should be essential continuity in Russian policy, though Yeltsin may bow to pressure to slow political and economic reform and to adopt a more nationalistic foreign policy.
''The political climate among the Russian elite has moved decidedly in a nationalistic direction over the past 24 months. Yeltsin will have to tack with that wind,'' says a senior Clinton administration official.
In another scenario, Yeltsin opts not to run for reelection.
In this case, the best outcome for the US could be the election of Viktor Chernomyrdin, who is now Russia's prime minister.
Mr. Chernomyrdin is well-known and respected as a political moderate by US officials. He announced last month that he would not run for the presidency, but his disclaimer is widely discounted. One Western observer equates Chernomyrdin with Gov. Pete Wilson (R) of California, who earlier this year threw his hat in the presidential ring despite frequent protestations that he would not.
More worrisome - if Yeltsin does not run or runs and loses - would be the election of almost any of the other leading figures on the Russian political scene, including Gen. Alexander Lebed. Mr. Lebed, a hero of the Soviet Union's war in Afghanistan during the 1980s, is a strong nationalist, although he has condemned Russian military action against rebel forces in the breakaway republic of Chechnya.
Another likely candidate is Gennadi Zyuganov, whose Communist Party is expected to make the biggest gains in next month's parliamentary elections.
The official says the administration is aware of but cannot confirm reports that parties close to Yeltsin, who would stand to lose if his reform program were slowed or repealed, are conspiring to prevent the presidential elections from taking place.
''It's sensible to take this contingency into account without necessarily assuming anything like this is going to happen,'' says the official.
Diplomatic analysts say no successor to Yeltsin is likely to change Russian policy on two issues deemed important to Washington: Moscow's determination to build nuclear reactors in Iran, despite strong US protests; and its opposition to expanding NATO eastward into central Europe.
''The Russians have already put themselves into a position that will serve their interests over the next two years or so: public opposition to enlargement, efforts to stymie it, but nothing to truly create a crisis with the US,'' says the Clinton official.
Where the change will be felt is in Russian's relations with the newly independent republics and in its domestic policy.
With respect to the former, diplomatic analysts say, look for more heavy-handedness in dealing with Russia's ''near abroad,'' led by efforts to ''Finlandize,'' or neutralize, Ukraine and possibly the Baltic republics.
''You run your economy, you have your language and flag, but on matters of foreign and security policy you defer to us,'' says Mr. Aron, paraphrasing what could become Moscow's message to Ukraine.
Another possibility: more active Russian intervention to block Western oil development in the Caspian Sea Basin.
With respect to domestic policy, the pace of political and economic liberalization might be slowed, though probably not reversed.
The issue is important to US officials, who believe that a government dedicated to domestic reform is less likely to pursue an aggressive foreign policy.
Public-opinion polls suggest that Russians don't want to turn the clock back. But they have wearied of reformers who say they must sacrifice now for greater good in the future.
''Russians are basically tired of that kind of shock therapy,'' says Gregory Guroff, acting director of the Center for Post-Soviet Studies, in Chevy Chase, Md.
Apart from the question of whether Yeltsin does or does not run is the issue of what role he will play in next month's parliamentary elections. If he is unable for health reasons to campaign actively, it is more likely that nationalists and especially Communists will score big wins.
On the other hand, ''if he's healthy enough to go on TV, he could help marginal, centrist political groups attain the 5 percent needed for representation in parliament,'' says Mr. Guroff.
No single party has more than 20 percent support in public-opinion polls. But huge Communist gains in recent local elections in Volgograd suggest that the party's popularity is rising in sync with Yeltsin's own decline in the polls.
Yeltsin was the ideal figure to dispose of the old regime but, analysts say, he has been less adroit at building a new Russia.
''Yeltsin knew how to clear the boulders and the rotten stumps but was somewhat at a loss as to how to sow and husband the field,'' says Aron, who is writing a biography of Yeltsin.
Only 5 or 6 percent of Russians now say they would support Yeltsin for reelection.
Many analysts fault the Clinton administration for placing too many eggs in the Yeltsin basket and not doing enough to develop relationships with other leading political figures in Russia.
''We have maintained contact with all significant political leaders in Russia,'' rejoins the administration official.