The transformation of Mikhail Gorbachev from the youthful but still uncertain figure of his Politburo years to a confident and powerful national leader has been so rapid and seemingly so effortless that it suprised even his most ardent boosters.

From the start, the 54-year-old Gorbachev appeared to be a man in a hurry who wanted the Soviet Union to set aside the traumatic and uncertain years of three Kremlin leadership transitions and who wanted to demonstrate that he was in this for the long run. Plunging into his work after taking office in March, he seemed to project an image of action and purpose wherever he went, urging the people to roll up their sleeves and make the Soviet Union over.

His activism, his instinct for maneuvering and, above all, his speechmaking abilities came across clearly on national television. He is the first Soviet leaderto be also a television personality, bringing with him a casual tone, a mistrust of verbosity and sloganeering, a self-deprecating wit and what many Soviets seem to sense as personal warmth.

His age alone was a factor. It immediately appeared to lift the nation's spirit. After a series of embarrassingly feeble old men at the top during the past four years, the sight of a youthful and energetic Kremlin leader seemed to symbolize the end of the interregnum and the uncertainties it bred.

Those disheartening years, starting with the last phase of Leonid Brezhnev's rule in the early 1980s, were marked by a loss of national confidence and the absence of reformist impulses. Gorbachev's accession to power, other things being equal, holds out the prospect of a significant modernization of Soviet society in the coming decade.

But other things are not equal. Quite apart from the magnitude of the task involved in national reconstruction of a highly centralized and hierarchical country, there remains the problem of a huge, entrenched party bureaucracy that is unlikely to succumb to new rhetoric and changing public moods.

Moreover, Gorbachev and his men have set the direction only of the new "economic mechanism"; they have yet to come up with specific policies on such intractable issues as prices, wages, fiscal incentives and managerial authority, not to speak of relations between enterprises and the state. Although the guideposts are set, they appear clearly inadequate in the absence of a rational and coherent plan of action.

Nevertheless, in the course of the past four months, Yuri Andropov's heir has emerged as a figure in his own right, a leader driven by ambition and determination to harness the country's potential and carry out a national reconstruction.

He seems to handle the job skillfully, more pragmatic on economic matters and lacking doctrinaire belief in the sanctity of tradition. He is concentrating on the future, clearly trying to disengage himself from a legacy of stereotypes.

Undoubtedly Gorbachev was fully aware of the importance of first impressions. In presenting his image to the country he may have been helped by the new and more sophisticated Soviet approach to public relations that has emerged during the past year. With television cameras following Gorbachev on his travels, the leader appeals directly to the people and reasons with them. To the outside world, this approach is epitomized by the more open attitude of the new government spokesman, Vladimir Lomeiko.

A more significant change than public relations, however, has been a revival of the impulse within the ruling elite to reshape the society. While the basic stalemate between the reformers and the bureaucracy remains, the country's political conditions have been transformed.

In this context, Gorbachev has a realistic opportunity to modernize the Soviet system, although most observers here believe that this cannot be accomplished without a major political struggle. The odds are that it will be an uphill struggle for Gorbachev. 'A Nice Smile, but Iron Teeth'

Gorbachev brought considerable skills and talents to his new job. Those who have dealt with him describe him as being a tough and competitive person with a gregarious approach and persuasive manner.

When Andrei Gromyko nominated him last March to succeed Konstantin Chernenko as general secretary of the Soviet Communist Party, he told the Central Committee, "Comrades, this man has a nice smile, but he's got iron teeth."

For the past four months Gorbachev has shown the country both his smile and his teeth.

During a trip to Leningrad, he was bantering with a jostling crowd in the street, seeking their support to "move the country forward." A woman in the crowd said, "Just get close to the people, and we'll not let you down." Hemmed in by the crowd, Gorbachev shot back, laughing, "Can I be any closer?"

Speaking at the Smolny Institute later that day, he posed a rhetorical question with an affable smile: Why not enjoy life, resting and lying at anchor? Why insist on moving the country forward?

No, comrades, he said, his face turning serious. "This is not the choice. We do not have such a choice." Just to maintain existing living standards and defense needs, he said, the country needed a minimal growth rate of 4 percent a year instead of the current 3 percent.

There was a populist tone in his speech when he talked of the need to raise living standards. But there was another and novel notion here: not slogans and propaganda, but the quality of life in the Soviet Union is ultimately the image the country presents to the world.

He also flashed some sharp teeth in the same speech. The program, he asserted, will have to be carried out. "We have no other options," he continued. "Those who do not intend to adjust and who are an obstacle to solving these new tasks must simply get out of the way." He added for emphasis, "Get out of the way. Don't be a hindrance."

In speech after speech he has hammered at the central theme of his pronouncements. The country, he has said repeatedly, must bring its economy to western levels of efficiency and quality. To pull the country from its long doldrums, he said, there must be a thoroughgoing change in the way the economy is run. The very "psychology of economic activity" must be altered.

In a relatively short time span, Gorbachev seems to have become more than a mere heir and executor of the Andropov political estate. He has acted more quickly and boldly in pushing his program and consolidating his power than any of his predecessors.

There was another novel thing in this. While he has absorbed much of the traditional party code, Gorbachev's instincts seem acutely contemporary and he appears to reflect the disquietude of the postwar generation -- he was too young himself to take part in the war -- and its longings.

All this does not mean a change in Kremlin goals. But it may indicate a change of means as well as a greater social concern, which Gorbachev has articulated repeatedly by talking about the lot of Soviet consumers and urging efforts to improve the quality of life in general.

Everyone here, naturally, favors a better life. The conflict is between those who maintain that this can be achieved by making the existing system work better and those who contend that the system itself must be radically modernized and adjusted to contemporary conditions.

For while everybody here speaks about the need for reform, a radical change would be resisted from below. Just an adjustment of the pricing system would be a revolutionary measure, for it would involve at least partial cuts in the huge government subsidies on basic food commodities, rents and transportation.

Although Gorbachev seems to favor radical changes in the future, he only has revealed his policies for what he has described as the "first phase" of his program. To spur the economy quickly, he has continued the campaign for discipline, introduced antidrinking laws and ordered a shift of investments into modernization of the existing industries. He has also proposed greater independence for individual enterprises and modest cuts in the number of nonproductive agencies that supervise the industry.

What the remainder of his program will be is as yet unclear. Broadening the Power Base

Whatever the shape of the program, its fate is most likely going to be decided during the coming seven months.

Depite the enormous powers of his office, a Soviet leader must have support throughout the party hierarchy to be able to institute any significant reforms. This means the people who wield real power in the provinces.

Gorbachev already dominates the Politburo and the Central Committee machinery in Moscow. But to be successful, he must have his people throughout this vast country who have to mobilize the population for exertions and sacrifices.

In moving quickly to consolidate his preeminence in Moscow, Gorbachev benefited from his position as the heir to Andropov, who initiated most of the economic and organizational changes now under way. Without Andropov's 15-month interlude as leader -- from November 1982 to February 1984 -- it would have been impossible for Gorbachev to move so quickly and on so many fronts.

The key men around the new leader, as far as policy is concerned, are Igor Ligachev, Nikolai Ryzhkov, Viktor Chebrikov and Vitali Vorotnikov, all handpicked and placed in high positions by Andropov.

There is ample evidence that personal relations between Andropov and Gorbachev were very close. They had met in the early 1970s, after Gorbachev became party chief of Stavropol oblast, a region in the south of Russia where Andropov's favorite resort city of Kislovodsk is located. According to well-placed officials, the older man liked the younger man's wit and intelligence and particularly the fact that there was not a whiff of corruption surrounding Gorbachev or his family. They frequently walked in the woods around Kislovodsk and discussed politics.

It was Andropov who recommended Gorbachev to Brezhnev in 1978 and proposed him for a position in the party secretariat. When Andropov became the Soviet leader there was no doubt that he was relying on Gorbachev and looked on him as a future leader.

That the old guard managed to block Gorbachev at the time of Andropov's death was a signal of internal resistance to changes. But the natural demise of the old guard and the positions of Andropov's men in the leadership made it seem inevitable at the time that Gorbachev was next in line after Chernenko.

When his time came, Gorbachev was in position to move with exceptional speed. In his first four months he managed to put four new people into the ruling Politburo, two of them -- Ryzhkov and Ligachev -- without going through the stage of being candidate or nonvoting members. (The other two, Chebrikov and Eduard Shevardnadze, 57, the Communist Party leader of Georgia, had been candidate members.) In doing so, Gorbachev seems to hold all the threads of power in his hands.

Ligachev, who is in charge of ideology and personnel, is now second in command to Gorbachev. Ryzhkov supervises the economy. Chebrikov runs the KGB, while Shevardnadze has replaced Gromyko as foreign minister, with Gromyko moving up to the post of titular head of state.

By moving his men into the key positions and at the same time ousting from the Politburo one of its strongest conservative members, Grigori Romanov, Gorbachev achieved unquestionable control of Kremlin power.

While essential, this control is not sufficient for the kind of changes Gorbachev is talking about. His power base and support must be far broader if he is to move the country forward, revamp the Stalinist economic system and change the mind set of the party.

During the coming months, Gorbachev and his men will have a chance to rejuvenate the policy-making Central Committee and the government and bring in younger and better educated people in leading local positions throughout the country. But all this has to be done before the next Communist Party congress, which is scheduled for February.

According to foreign and Soviet observers, Gorbachev will have a realistic chance to embark on the path of national reconstruction only if he manages to replace the local cadres before the party congress. A failure to do so, on the other hand, would mean that internal political differences would linger and that they would gradually sap the strength of the reformist trend. Emphasis on Soviet Bloc Cohesion

Gorbachev's preoccupation with internal economic and political issues has been such that his attitude on foreign affairs remains as yet unclear.

Like his predecessors', his top priorities are the Warsaw Pact allies and China. One can expect from Gorbachev a greater emphasis on the cohesion of the Soviet Bloc. On China, more than any of his four predecessors, he has voiced firm hopes that Sino-Soviet relations would be normalized and has suggested that he was prepared to go the extra mile to achieve this.

In general, his administration has shown signs of flexibility and realism with respect to Europe, Asia and other parts of the world and the willingness to recognize facts as they are. This is reflected in Moscow's attitude to the European Community and in its proposal for a pan-Asian security conference.

Gorbachev's attitude toward the United States seems more conventional. Like his predecessors, he has argued that the Soviet Union had tried for a long time to develop friendly relations with the United States but that the Americans have not responded in kind. Specifically, he has charged that it was the United States that abandoned detente and meaningful arms control efforts, seeking instead to achieve military superiority over the Soviet Union.

While Chernenko's policy remains in effect, Gorbachev threatened recently to break off the Geneva arms talks if the Reagan administration does not take a "more reasonable" position at the negotiating table.

But there is a clear impression that Gorbachev wants to reach some sort of accommodation with the United States that would limit the scope of the arms race. His readiness to meet President Reagan next November seems to point in that direction.

Another distinct impression is that Gorbachev's interest in foreign affairs at this point is primarily linked to his domestic policies as the continued arms buildup and other foreign commitments would inevitably interfere with his plans to modernize Soviet society and improve living standards.

In one of his speeches, Gorbachev has said that the two great nations, with different social systems confronting each other across the world, should have a "civilized" relationship to avoid a nuclear collision. The Geneva summit may help clarify what he had in mind.