Only four years ago, supporters of Walter F. Mondale's presidential candidacy warned that reelection of President Reagan would give him a window of opportunity to mold the Supreme Court in his own image by replacing five justices then more than 75 years old.

Now, the window is closing rapidly because of politics in an election year and the time needed to process nominations, and Reagan has been able to replace only the two most conservative members of that group, former chief justice Warren E. Burger and former associate justice Lewis F. Powell Jr.

Reagan's three appointees -- Sandra Day O'Connor, who replaced the late Potter Stewart after his retirement at age 66; Antonin Scalia, and Anthony M. Kennedy -- have nudged the court to the right but likely will not produce the ideological shift long sought by Reagan backers.

The remaining members of the quintet are William J. Brennan Jr., 81, Thurgood Marshall, 79, and Harry A. Blackmun, 79, who appear to be in no hurry to retire.

Justice Byron R. White, 70, says he has no plans to resign, despite rumors that White, who generally votes with the court's conservative wing, is going to announce his departure at the end of this term.

Some Senate sources say Reagan's window has already closed. Others say that, if an opening occurs in the next two to four weeks, he might win a confirmation vote depending on whose seat was being filled and whether Reagan's nominee would alter the court's ideological balance.

Some administration officials say they believe that confirmation of a new justice is possible as late as the Democratic National Convention July 15-18. Senate aides representing members of both parties say that is unlikely.

The Democratic-controlled Senate is thought almost certain to trot out an opposition letter to Reagan, the way the GOP opposition did to then-President Lyndon B. Johnson's nomination of Justice Abe Fortas to be chief justice in 1968.

The letter, dated June 26, 1968, and signed by then-senator, now White House chief of staff Howard H. Baker Jr., said "any nominees for vacancies on the Supreme Court should be selected by the newly elected president . . . after the people have expressed themselves in November's elections."

Congressional sources say the Senate Judiciary Committee, where hearings on court nominess begin, would not dream of considering a high-court nominee after April unless he or she were someone that Reagan does not want to see approved.

While a full transformation of the Supreme Court may elude Reagan, it appears likely that he will leave a federal judiciary firmly controlled by staunch conservatives.

Reagan has appointed 45 percent of the nation's 743 federal district and appeals court judges and will almost certainly have appointed more than half of them before he is to leave office next January.

Another 27 nominees are pending before the Judiciary Committee, and there are about two dozen other vacancies for which the administration has not nominated anyone.

Senate Judiciary Committee Chairman Joseph R. Biden Jr. (D-Del.) has said he intends "to give everyone that has been brought before us an opportunity to be voted up or down" and has set no deadline.

In reality, however, the window for nominations to appeals and district court vacancies shuts by June. After that, the national political conventions, campaigns, Congress' summer recess and the November elections would make it difficult for confirmation of all but a handful of nominees, those who enjoy the favor of powerful senators.

That means that noncontroversial nominees before the committee or whose hearings come up soon will likely be confirmed. But new nominees considered controversial, for reasons of qualifications or ideology, likely would have a difficult time surviving the committee.