Jimmy Carter talked about it in 1980. Walter F. Mondale talked about it in 1984. Interest groups across the political spectrum -- from Planned Parenthood to the Moral Majority -- variously dreaded or heralded a revolutionary transformation of the Supreme Court under Ronald Reagan. As a simple matter of actuarial fact, President Reagan was believed likely to have the opportunity to make five appointments to the aging court.

But now, entering the sixth year of the Reagan era, time is beginning to run out. Reagan, who has nominated only one justice, Sandra Day O'Connor, needs at least one more conservative to replace a moderate or liberal vote in order to effect a substantial shift on the court. The wholesale judicial revolution he seeks would require two such changes.

Reagan has, at most, two more years to put his mark on the high court. And with control of the Senate at stake in this year's elections, he may have considerably less time than that.

Some observers, citing Lyndon B. Johnson's inability to elevate Abe Fortas to chief justice in the last year of his presidency, say it will be difficult for Reagan to put anyone on the bench in 1988 even if the Republicans retain control of the Senate. If the Democrats prevail in November, it will be virtually impossible to fill a high court vacancy in 1988, and difficult even in late 1987.

A senator tapped for the court might squeeze through in early 1988, some observers say, given the traditional Senate support for its own, but no other nominee would stand much of a chance with the Democrats in control.

There is even talk that Senate Democrats would stall a nomination as early as this spring, hoping to regain power in the fall. In that event, Reagan's shrinking "window of opportunity" for a judicial revolution could be reduced to the first six months of 1987.

All this assumes that Reagan will have more vacancies to fill -- a major assumption given the remarkable staying power of the current justices.

The average age of the justices -- 70.9 years this month -- is slightly below the average age of the "Nine Old Men" who blocked Franklin D. Roosevelt's New Deal in the mid-1930s. But in terms of median age, the Burger court is the oldest in history, with five of its nine members over 77 years old.

The court, according to a recent study in Judicature, a legal magazine, is the most stable since 1823, with only one change of membership in the last 10 years. No sitting justices appear to be talking about leaving voluntarily.

The court's liberal wing, anchored by Justices William J. Brennan Jr., 79, and Thurgood Marshall, 77, is apparently not even thinking of resigning.

Centrist Justice Lewis F. Powell Jr., 78, who was hospitalized last year for removal of a cancerous prostate, plans to stay as long as his doctors say he can; Harry A. Blackmun, 77, another member of the center group, has not indicated any intention of stepping down.

Chief Justice Warren E. Burger, 78, a member of the conservative wing, is likely to stay on for at least this court term and another.

If there is a vacancy this year or next, the key variables would be whose seat is vacant, who is nominated and when the vacancy occurs.

For example, if centrist-conservative Justice Byron R. White, a relatively youthful 68-year-old, decides to retire, and Reagan were to nominate one of two highly regarded conservative appeals court judges, Robert H. Bork or Antonin Scalia, to replace him, even a Democratic Senate would probably confirm either of them, even in late 1987.

On the other hand, should Marshall leave and either Bork or Scalia be chosen to replace him, a Democratic Senate could block the move, certainly in 1988 and possibly in 1987.

A Republican-controlled Senate would probably prevail in such a scenario in 1987, but not without considerable turmoil.

The Judicature study noted that the Senate has approved high court nominees about 80 percent of the time. Twenty-five have been rejected or postponed while 113 have been approved.

But the approval rate falls to 25 percent, according to the study, in the last year of a presidency when the Senate is controlled by the opposition.

Given the highly ideological approach the Reagan administration has taken to judicial appointments, Senate Democrats are likely to be especially reluctant to defer to Reagan's wishes, especially if the vacancy seems likely to tip the ideological balance on the court.

If the Senate goes to the Democrats, one key Democratic staffer said, the Reagan revolution may never happen. The administration would be able to fill any vacancies that might occur in early 1987, he said, but the Senate would not be likely to approve nominees who would reshape the high court.