CONSTANTIN Chernenko was a sick man -- perhaps that is why he was chosen -- when he was elevated to the top spot in the Kremlin last February. Since then this 72-year-old heart patient, who has no achievement or even aspiration connected to his name, evidently has gone downhill. He is reported to have been in a Moscow hospital since July, and whether he is conducting his office at all is in some doubt. That doubt is only aggravated when boilerplate statements are repeatedly issued in his name -- the "interview" published yesterday falls into that category -- while he remains unseen.

What this means from a political standpoint is that the Kremlin elite, which is unaccountable in these matters, is quite possibly on the verge of installing its fourth chief -- after Brezhnev, Andropov and Chernenko -- in less than two years. The old guard has steadily refused to repose power in the hands of a younger man, who presumably would have the requisite time and energy to put his own mark on the structure of Soviet authority. What tension there is to the Chernenko countdown arises precisely from the question of whether the likeliest younger man, 53-year-old Mikhail Gorbachev, will make it to the top this time.

Has it made a difference that the leading post in the Kremlin has been occupied through most or all of the Reagan presidency by a dying man? The administration has often blamed some part of the dismal state of Soviet-American relations on the lack of a strong and active Soviet leader. It has also been putting forward an election-year theory that the Soviets have now had time to absorb the lessons President Reagan has been trying to teach them by his arms policies and assertion of American will and that, being thus chastened, they may be ready to do business with the United States in a second Reagan term.

The self-serving quality of this theory is evident. Still, the Soviet system, being inherently cautious and bureaucratic, needs a strong push from the top to take risky initiatives -- and all peaceful initiatives are risky. It would be startling if the Kremlin had not been slowed in its political responses by the lack of such a strong push since the late Brezhnev years.

The most conspicuous acts of Soviet policy in the past few years have appeared to reflect a conservative consensus or simple negative reflex -- cutting off Solidarity, for instance, rearming Syria, shooting down the Korean airliner, boycotting the arms talks and the Olympics. All these acts have cost Moscow heavily in its dealings with the United States. They have also helped to build popular support for a hard- line, anti-Soviet president whose policies Moscow professes to find anathema. Some strategy.