Moscow after Chernenko is bound to be a lively place. Washington after Chernenko is more subdued. The town is a bit on edge, waiting to feel out Mikhail Gorbachev and sensing in the accession of a younger and more vigorous man, one who is going to be around for a while, that something important in the whole Soviet-American equation has changed.

So it has. The easy life is over. Even while the Reagan team complained that the Kremlin's turnover and decrepitude denied the United States an interlocutor, it enjoyed the fact and appearance of weak Soviet leadership. The Soviet political system, by keeping a series of dying old men in high office, was proving the administration's basic ideological contention that communism is bankrupt -- a system run by a selfish elite and fit only for the ashheap of history, as President Reagan put it. A system that can elevate a 54-year-old comer may be no less bankrupt, but you can't prove it so easily.

Meanwhile, three Soviet successions in 28 months have drawn American attention from the ostensibly permanent and determining essences of the Soviet system to the personal variables of its leaders. There are certain comforts in dealing with a known system, no matter how bankrupt and "evil" you believe it to be. These have been replaced by the uncertainties of dealing with a particular person, Gorbachev, who, even before he has done anything, has been endowed with a capacity to do more than his predecessors of the last decade.

It is worth recalling the quiet little jolt that Yuri Andropov gave Washington when, taking over from Leonid Brezhnev, he threw out some hints of flexibility in a few sticky foreign policy areas and launched a campaign of anti-corruption and workplace discipline. Andropov lasted only for a few months, but the point is that, for all the vaunted immobilism of the system, there is always room for initiatives, for certain quick fixes, by the new man on the Soviet scene.

There is also room for darker clouds to spread soon over the American scene. Budget deficits pushing past $200 billion and trade imbalances pushing toward that level have provoked widespread fears -- though not, evidently, in the Oval Office -- of economic retribution. This could take some of the edge off the administration's easy assumptions of the naturalness of American progress and the superiority of the American way.

The Soviet change, moreover, costs Reagan overnight a subtle psychological advantage that has flowed from past comparisons of Soviet and American leadership. He has been, by and large, the bright exception to the rule of fading 70-year-olds. Now he may be seen as a man who is rather old for his job, faced by a man of appropriate years.

But there is more, in the realm of what Marxists call the "objective" forces. Just a few days ago, the administration appeared -- to itself, certainly -- to be in a position of hard-won and unprecedented advantage. Technologically, economically and politically, it was on a roll, entering a critical forum -- the Geneva talks -- at a moment when the Soviet Union was lagging, though far from crippled, in all those categories. In nearly 20 years Washington had not come up to a negotiation with more reason for confidence.

Broadly, its choice was whether to step on the gas and attempt to set the evident American advantage in strategic concrete, with an agreement if possible, without one if necessary, or whether to throttle back a bit and offer Moscow an arrangement that, if it "took," would reflect a certain ragged but agreed parity and would head in the direction of a modest general settling down. The administration's stated detemination to push "Star Wars" to the hilt seemed to me to point toward the first choce.

If there were reason to question the administration's approach earlier, there is further reason now that Gorbachev has arrived. Any American inclination to take advantage of a time of troubles for Moscow needs to be meas possibility that Moscow's troubles may not be so disabling after all.

In the 1970s, Arnold Horelick observes, the United States was distracted, the Soviet Union got greedy, and this produced an American backlash: Ronald Reagan. It could happen in reverse, he warns: "don't kick a superpower when it is down." With the measure of change now possible in Moscow, that warning must be updated a bit: don't kick a superpower when it may be getting up.

I don't see that the administration has yet come to this sort of review. But it does seem to me the main task that Gorbachev's promotion poses to Washington.