Leonid Brezhnev, a tired, tough old walrus, hasn't yet been dislodged from the Soviet icecap. But he is being crowded by the younger bulls who dare not challenge him separately but threaten him collectively.

This is the assessment of the Central Intelligence Agency, whose top-secret reports portray Brezhnev as a wary but wily old manipulator who has stayed on top by outmaneuvering his rivals.

If he can't be pushed aside just yet, he will give ground to underlings who gather enough strength to budge him. He pulled back from detente, for example, under pressure from the hardheads who prodded him into authorizing the Afghanistan aggression.

Intelligence reports suggest the move into Afghanistan may be part of a strategic offensive aimed at securing a stranglehold on the West's lifelines. If the Russians should gain control of the oil routes, they could bring the Western industrial nations to their knees. The effort will intensify, intelligence sources predict, as Brezhnev weakens. Illness and exhaustion have already sapped him of his vitality. He disappears for weeks at a time to recuperate from his various ailments. Even when his health holds up, he is seldom able to spend more than three hours a day at his desk.

Yet at 73, he is middled-aged by Politburo standards. Five of the 14 ruling members are older than he is. The dean of the Polithburo, Arvid Pelshe, will be 81 years old this month. Still, the power structure is controlled by these aged men. The six senior members, reports a national intelligence bulletin, "function as a small inner collective within the Politburo."

This has caused frustration at the lower levels of the Kremlin, particularly among the younger Politburo members who feel stifled. "There is little or no devolution of authority to the juniors on the Politburo," declares the intelligence bulletin. They have been held down by Brezhnev, who has always been able to outmaneuver any potential rivals before they gained enough power to threaten him.

Four years ago, the impatient, ambitious younger leaders succeeded in stirring up some mild criticism of Brezhnev's "cult of personality." The CIA picked up reports, according to one secret dispatch, that "Soviet party members in several institutes have written letters complaining of a budding Brezhnev cult. The tenor of these letters reportedly was that the party had suffered under Stalin's personality cult and Khruschev's 'subjectivism' and should not allow such errors to be repeated under Brezhnev."

This campaign was apparently inspired by some junior Politburo members. "It is not clear," related an April 15, 1976, intelligence report, "whether any senior member of the Soviet leadership is critical of Brezhnev's toleration of -- if not fondness for -- personal acclaim."

The ancient and ascetic Mikhail Suslov, the Kremlin's ideological witch doctor, was suspected. But Brezhenev's hand has always been quicker than any rival's eye. He not only suppressed the criticism; he wound up increasing his power: in June 1978 the Supreme Soviet gathered in the Grand Kremlin Palace to install Brezhnev, already the party leader, as head of state as well.

The intelligence reports suggest that the senior Politburo members rallied around Brezhnev for their own preservation. States a top-secret report: "Those of his generation in the senior leadership would probably like him to stay on so as to preserve their own positions."

Beneath the Kremlin's opaque surface, meanwhile, the undercurrents are seething. There is "no lack of aspirants," declares a secret analysis, for the policy-making seats. The younger leaders "know that they will rise only in part because of what they know as opposed to whom they know, for some of the most important Soviet-style political battles are fought on the patronage front . . . . "

Concludes another report: "There is still no sign of preparations for succession when Brezhnev eventually leaves office."