And now Konstantin U. Chernenko

Soviet Communist Party Secretary Konstantin U. Chernenko, the new leader in the Kremlin, owes his political career to the late head of the Soviet state, Leonid I. Brezhnev. But for some reason Mr. Brezhnev used his protege essentially as a glorified paper pusher and aide-de-camp for many years. Mr. Chernenko operated behind the scenes and did not seem to be terribly influential.

Then Brezhnev's health began to decline. He appeared to be uneasy about old allies, such as Andrei P. Kirilenko, who gained greater prominence at the ailing leader's expense. That is how Chernenko got his chance to join the Politburo. He was a loyalist. In addition, he was sufficiently obscure not to represent a personal threat to Brezhnev, who was showing signs of senility.

It is no wonder that some of Chernenko's Politburo colleagues initially treated him with contempt. He had few accomplishments except gaining the confidence of an invalid. He did not rise through the ranks in the accepted fashion. He did not occupy important positions that would test his abilities. He was a beneficiary of Brezhnev's embarrassing disabilities - the disabilities that left the Soviet Union without firm guidance from the top.

Yet, helping Brezhnev walk straight is no proof that one can keep the nation on course. Hence, the Soviet ruling group opted for Yuri V. Andropov, a man not identified with Brezhnev and who enjoyed the image of a shrewd, decisive, and even brutal politician.

Andropov's death 15 months later has given Chernenko a second chance. Outsiders cannot be sure exactly why the Politburo has selected him over some younger candidate such as Mikhail S. Gorbachev, 52, or Grigory V. Romanov, 61. And if the Politburo elders were determined to keep the power in the hands of the old guard, they could still have picked someone more authoritative and with a more distinguished record, such as Defense Minister Dmitri F. Ustinov, 75.

So why Chernenko? One obvious answer is that no younger candidate commanded sufficient support and that the only way to avoid a destructive struggle was to select a safe caretaker, someone who would maintain the leadership and the country in a holding pattern while more formidable contenders sorted out their differences.

Is it possible that since Brezhnev's death members of the Politburo have discovered something that has modified their initial reluctance to entrust him with power?

I believe the answer is yes. For the first time in his political career Chernenko has genuinely been his own man. And as far as members of the leadership are concerned he has performed his own duties with at least a passing mark. Despite the obvious disappointment that he was passed over in favor of Andropov, Chernenko played carefully by the rules. In the name of presenting a facade of unified leadership to the world, he agreed to nominate his rival for the general secretaryship. He also officially recommended Andropov to be elected chairman of the Supreme Soviet Presidium.

While Andropov was busy consolidating his power, Chernenko behaved like a good soldier. He did not maintain a high profile. He carefully avoided creating the impression that he was still an active candidate.

With Andropov's illness, Chernenko began to be treated as a party leader second only to the absent Andropov. While Western analysts were preoccupied with the addition to the Politburo of supposed loyalists to Andropov, it was evident from Soviet media reports that Chernenko was substituting for Andropov. And when he passed on, Chernenko was formally given the duties he has been performing de facto for some time.

That means there is no new leadership in the Soviet Union. Andropov's death eliminated an element of uncertainty. But the same group with the same division of labor is in charge. No immediate major change in Soviet policies is in sight, either at home or abroad.

It may very well be that Mr. Chernenko will resign himself to reigning without ruling. And then his tenure, like Andropov's, will become nothing more than a footnote to Soviet history. There is also the possibility, however, that a place on the top of the Soviet hierarchy will encourage the new general secretary to demonstrate new qualities of statesmanship.

For the United States, Chernenko's ascension to power may offer some opportunities. But there is no reason to expect that the Kremlin will suddenly become friendly to Ronald Reagan. A summit meeting with Mr. Reagan during the US election year would require a lot more than the administration seems willing to offer.

It may be tempting for Mr. Chernenko to establish his political legitimacy quickly. To do it on the domestic front without antagonizing powerful interest groups is going to be difficult. Rapprochement with the US would not be as controversial among the Soviet elite as would meaningful economic reform. And Chernenko has usually sounded somewhat more conciliatory to America and the West than most of his Politburo colleagues.

This provides a possible opening to the Reagan administration. No one can predict whether the Soviets will respond to any American overtures that are short of major concessions. And major concessions are certainly not in order just because another man has been confirmed as head of the rival superpower.

Still, Andropov's replacement - even if he does not change much substantively - provides the US administration with a good excuse to communicate its interest in normalization to the Politburo. But for this message to be taken seriously in Moscow, it has to be delivered subtly and tactfully, and certainly without the insistence that the Soviets are obliged to reciprocate right away.