Mikhail Gorbachev's swift ascent to power presents the 54-year-old politician with a daunting agenda of long-term domestic problems that have confounded his predecessors and the prospect of having enough time at the Soviet helm to do something about them.

Since Leonid Brezhnev's healthbegan to decline nearly a decade ago, the Kremlin has had to focus on the leadership stakes and succession. Momentum and the ability to implement long-term planning decisions have been lacking, experts on the Soviet Union believe.

"Those days seem to be past, for a while," said one U.S. diplomat in Washington. "The new leadership may now be able to offer new direction or bring any new decisions. But at least they should bring some fresh energy."

Moscow already has signaled that the change in leadership should bring no immediate changes in policy. But Kremlinologists interviewed yesterday expect that Gorbachev will move to establish his authority early in several areas of domestic policy, ranging from economic and agricultural planning to raising the general level of Soviet morale and rejuvenating the bureaucracy.

Despite attempts by both Brezhnev, and his successor, Yuri Andropov, to bring about some economic reforms, the Soviets long have lacked a leader who could push forward over the long term with forceful, new policies. Consequently, economic growth, dramatically down from peaks of two decades ago, has dragged.

Even the limited economic experiments attempted during Andropov's short reign have not been very thoroughly implemented. Ed Hewett, an economist for the Brookings Institution, explained why: "No strong leadership has been nudging them along."

The new Soviet leader already has shown an impatience for the sluggish bureaucracy and a general support for wage reform -- both big concerns of Andropov's. "I expect him to push on both those issues, and some others over the next few months," Hewett said.

Others consider it doubtful that Gorbachev will stop at minor economic reforms.

No long-range economic experiments have been introduced since 1965. Andropov's brief efforts concentrated on setting the stage for broad reforms by persuading and exhorting the Soviet populace to work harder and to drive corruption out of the system. His death in Febuary 1984 halted longer term planning.

In his maiden speech as new Soviet leader Monday, Gorbachev stressed the need for economic changes. Specialists on the Soviet economy pointed out yesterday that the five-year economic development plan due to be introduced at the Soviet Communist Party congress next winter would be the primary vehicle for achieving such changes. They predicted that Gorbachev will involve himself even more deeply in the adaptations of that plan that already are under way.

One of the key economic policies under scrutiny, according to one U.S. Soviet specialist, is agricultural policy. Soviet farm output has dropped to record levels in the past few years. Jerry Hough, a Kremlinologist with the Brookings Institution, said he believes that decreasing Soviet dependence on a grain economy is probably at the top of the new leadership's agenda.

Gorbachev served as minister in charge of agricultural policy since 1979 -- some of the worst years of agricultural output in Soviet history. But Kremlinogists here point out that he is not likely to be blamed for such poor performance. He will be able to argue that low farm production was due in large part to poor weather. Moreover, Gorbachev was, as a Politburo member, essentially carrying out the policies formulated by Soviet leaders Brezhnev and Andropov.

In addition to economic reform, Soviet experts here expect some new impulses in Soviet relations with the East Bloc countries.

Several leaders in those countries have suggested recently that the Kremlin leadership crisis has resulted in mixed signals between Moscow and bloc capitals. Moscow's East European allies are likely to get a chance to evaluate Gorbachev as the supreme Soviet leader at an important Warsaw Pact meeting that State Department experts predict will be held by May.

The 30-year pact binding the seven countries in mutual defense arrangements is due to expire in May.

A session to discuss and renew the pact, originally scheduled for January in Sofia, Bulgaria, was canceled, due to the illness of Gorbachev's predecessor, Konstantin Chernenko. The agreement would lapse if not ratified.

Experts also expect that the new leadership will make efforts to reverse what several western observers have described as a declining morale among Soviet civilians. "There is a certain amount of stagnation that may well be one of the symptoms of an aging, ailing leadership," said Arnold Horelick, a Soviet specialist with the Rand Corp. and UCLA in a telephone interview. "Civic morale is at a very low level. It's a society that shows signs of being unhealthy."

Horelick predicted that Gorbachev will move first in this area. "I think he'll do his best to get people excited and get the system moving again in the style of John Kennedy," said Horelick. "And there's such a thirst for freshness in Soviet society, his attempts may well resonate."

However, Horelick pointed out that the age and sicknesses of Soviet leaders alone cannot carry the blame for stagnation and the mounting backlog of unfinished business. "The system does not encourage spontaneity, flexibility and a fast-paced style of government," he said. "Whether Gorbachev and others can change all that very quickly is doubtful."