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The new Soviet leader, Mikhail Gorbachev, has opportunities perhaps unparalleled in recent Soviet history. Because of a unique set of circumstances -- some of them historical coincidences, some a result of Mr. Gorbachev's own political acumen -- he has a chance to quickly place his own stamp on the ruling Communist Party, and, therefore, on every aspect of this country's political and governmental system.

It is even possible that the normal period for a Soviet leader to ``consolidate power'' -- which one Soviet expert says usually takes about five years -- could be shortened, and he could emerge as an effective Soviet leader in relatively short order.

``It is an extraordinary opportunity,'' says a Western diplomat.

Some of the reasons for that are printed on the calendar.

The most obvious one is Gorbachev's age -- 54. He is the youngest Soviet leader to take office since Joseph Stalin and suffers from no discernible health problems.

Within months, he will preside over a previously scheduled Communist Party congress -- a once-in-five-years event of undoubted political significance.

In the run-up to it, and at the congress itself (which most analysts think will take place in November), Gorbachev will have abundant opportunities to place his supporters in key positions.

There will be at least one meeting of the Communist Party Central Committee, which could theoretically approve new appointments to the Politburo.

And the entire Central Committee itself will be up for election at the congress. Additionally, the congress is scheduled to approve a new party program -- giving Gorbachev the chance to jettison an embarrassingly inaccurate program adopted in 1961.

The program, among other things, promised that the average Soviet worker would surpass American workers in per capita production, and that the party and government would provide a solution to this country's housing problem by the 1970s. Neither came about.

Too, sometime shortly before or after the congress, the Supreme Soviet -- the country's nominal parliament -- could meet to approve changes in governmental personnel as well.

These opportunities to make appointments come, moreover, at a time when vacancies are occurring naturally, instead of having to be forced. Politburo ranks have been thinned by recent deaths to only 10 full members, down from 16 in 1976, and there have been no new appointments in over a year. There is also the strong possibility of change in government ranks, since Prime Minister Nikolai Tikhonov has, according to diplomats, made no secret of his wish to retire soon.

Gorbachev is already well-situated to influence the outcome of these events. The choice of new members of the Central Committee is largely controlled by the party Secretariat and Politburo. Gorbachev heads both bodies. They will approve a single slate of candidates in advance, and submit it to the congress for pro-forma approval. The delegates to the congress are expected to have little actual say in the matter. And, for good measure, the Central Committee secretary supervising the elections for delegates to the congress -- Yegor Ligachev -- is a firm ally of Gorbachev. (In fact, Mr. Ligachev himself is considered a likely Politburo appointee by some analysts.)

Taken together, this means that, ``he's got a near-term opportunity to put in some fresh faces'' into key posts, says a Western analyst.

Moreover, Western analysts say his speedy choice as General Secretary -- the announcment was made less than five hours after Konstantin Chernenko's death was announced last Monday -- means Gorbachev faced no serious opposition within the party leadership.

What about the military and the Soviet secret police, the KGB?

``I don't think he has a problem with the KGB, in that the KGB saw him as the chosen successor'' of former KGB head and party leader Yuri Andropov says a Western diplomat.

The present KGB chief, Viktor Chebrikov, is also apparently a close ally of Gorbachev.

Gorbachev's relation with the Soviet military is less clear. He has made the ritual promises of giving the military everything it needs to provide for the country's defense, but his ties with senior military officials are unknown.

Western analysts are still unable to come up with a clear explanation of a major shakeup in the military last year, which saw the chief-of-staff, Marshal Nikolai Ogarkov, demoted. They do not know what role, if any, Gorbachev played in the matter.

The military also lost a voice in the upper reaches of the party with the passing of the Soviet Defense Minister, Dmitri Ustinov, last December. His successor, Sergei Sokolov, lacks the stature of Marshal Ustinov, and is not even a member of the Politburo.

There are persistent, though unsubstantiated reports here that Marshal Sokolov, 73, is himself ailing, and may soon have to be replaced.

But, as a Western diplomat says, ``it's an open question as to whether a professional military person will be elevated to the Politburo.''

In the meantime, the chief spokesman for defense on the Politburo appears to be 63-year-old Grigory Romanov, Gorbachev's erstwhile rival for the party leadership.

But Mr. Romanov apparently did not have the support to make a serious challenge to Gorbachev.

And now, as a Western diplomat concluded, ``He will probably make it his business to get along with the general secretary.''

To be sure, Gorbachev still faces some potential stumbling blocks.

For one, there is the collective nature of decision-making in this country, centered in the Politburo. And that Politburo is, with the obvious absence of Mr. Chernenko, the same one that ruled this country before Gorbachev came to power.

``It's essentially the same cast of characters that's been running the show,'' says a Western analyst.

``Gorbachev was part of the collective then, and he's part of the collective now.''

And the older members of the Politburo, says another analyst, ``will serve as a check on the too-rapid accumulation of his personal power.''

``Since Stalin, there's been a tendency to do that,'' the analyst says.

Stalin, who was general secretary from 1922 to his passing in 1953, ordered the death of untold numbers of Soviet citizens during his years in power.