The Kremlin will find it hard to replace Mikhail Suslov.
The indirect effects of his passing Jan. 25 are likely to be substantial, even if they are not immediately visible in Soviet policy.
Although the No. 2 man in the Kremlin, Suslov in some respects was more influential than President Brezhnev.
A member of the Soviet leadership since Harry Truman was in the White House, and since Brezhnev was a regional party leader in the Ukraine, Suslov could claim unparalled experience at the center of power.
A series of recent interviews by the Monitor with senior Soviet officials suggests that in addition to his Politburo influence Suslov had a more direct role in the day-to-day running of this superpower than did Brezhnev. (The interviews, including some with officials who worked with Suslov, were for a series of articles on Soviet politics and power to appear in the Monitor next month.)
In addition, Suslov played an important role in the transition from the discredited leadership of Nikita Khrushchev to the rule of Brezhnev. And foreign diplomats here assume that the transition from the ''Brezhnev era'' will be a more difficult process for the Soviets without Suslov. (There are widely published, if ultimately unconfirmable, reports that Suslov turned down the post Brezhnev now holds.)
Meanwhile, his disappearance from the scene may bring the Soviet leadership to do something it resolutely resisted at the five-yearly Communist Party Congress here last February: to add to, or otherwise rearrange, the roster of elderly men at the top.
In at least one sense, Suslov is irreplaceable. Russians, even Soviet Russians, have an inbred respect for age and experience, a fondness for father figures. Mikhail Suslov fit the bill. He was the only one of the current members of the Politburo who had walked the corridors of power since Stalin's time.
For Suslov, the ''Polish crisis'' was but one in a line, a long line including upheavals in East Germany (1953), Hungary (1956), and Czechoslovakia 12 years later. He was the only member of the present leadership in a position to talk to Mr. Brezhnev as an equal, even, conceivably, as a superior.
His practical influence is a question complicated by Soviet secrecy. (Typically, word of Mr. Suslov's passing Jan. 25 reached foreigners here only around noon the next day. Typically, also, many Muscovites knew something serious was afoot when a popular, early morning satirical show on Moscow radio was inexplicably replaced by somber music.)
But officially published information on Suslov's functions and speeches, together with the Monitor interviews, provides some pointers.
At the time the interviews were conducted, both Suslov and Brezhnev were among only six people in a nation of some 268 million who held seats on the Politburo and on the Central Committee Secretariat. Officially, the Secretariat is supposed to ''direct current work, chiefly the selection of (party) cadres and verification of the fulfillment of party (that is, Politburo) decisions.''
This, it does. But senior officials suggested that the role of the Secretariat is wider than the official brief implies. Although genuinely junior to the Politburo, the Secretariat was said often to make, or formulate, major policy decisions - particularly, but not exclusively, on domestic issues. Domestically, as its official job description suggests, the Secretariat plays the major role in the nuts-and-bolts questions of governing the USSR.
Brezhnev, as party general secretary, is in charge of the Secretariat. But various officials said he does ''not necessarily'' attend its regular meetings. The officials implied Brezhnev was often absent. When he was, they said, Suslov presided.
That role will presumably now fall to either Andrei Kirilenko (Brezhnev's No. 2 for formal state functions) or longtime Brezhnev aide Konstantin Chernenko. Officials said either of these two men presided over the Secretariat when both the President and Suslov were absent.
In the inexact ''science'' of Kremlinology, the Secretariat role of Kirilenko and Chernenko, both mentioned by foreign analysts as possible successors to President Brezhnev, clearly bears watching.
Beyond Suslov's position on the Secretariat, he was the senior Soviet authority on questions of ideology. No doubt for this and other reasons, it was he, among top Politburo members, who received the Polish foreign minister on a post-martial-law visit to Moscow earlier this month.
It was Suslov who, last spring, keynoted a national ideological conference here. It was he, in his last major published statement, who opened a conference of Soviet social scientists with a wide-ranging policy address in October. The man thought most likely to succeed him as a chief ideological spokesman is Boris Ponomaryov, a candidate (nonvoting) member of the Politburo since 1972.
In journalistic and Kremlinological cliche, Mikhail Suslov was the quintessential ''hard-liner'' (a label also routinely applied to Ponomaryov). He was described as a walking file cabinet on orthodox Marxism.
It was often assumed, for instance, that Suslov might well have been whispering in President Brezhnev's ear that the proper reply to Polish reform was Soviet tanks.
This, we do not know. But there is some reason to believe that dubbing Suslov a simple ''hard-liner'' - though not necessarily unjust - affords, at least, an incomplete portrait. It is true that Khrushchev's memoirs say Suslov opposed rapprochement with Yugoslavia because its economy was ''not really socialist.''
Yet he is also said, in 1957, to have sided with Khrushchev against a bid by Stalin-era figures to oust him. As for Poland, all we know for now is that the Politburo, even with Suslov, did not roll out the tanks.
In his October speech, Suslov spoke very toughly on Poland and very warmly about detente. At the April ideological conference, he said there must be ''no compromise in the struggle of ideas . . . with our (Western) class enemy.''
He also said that, at home, the party risked substituting ''political twaddle'' for action. Soviet social science must be freed from ''elements of scholasticism and dogmatism.''