An odd thing happened on the way to the historic Supreme Court decision expected yesterday on the constitutionality of the Gramm-Rudman-Hollings budget-balancing law.

All Washington was ready. Rep. Mike Synar (D-Okla.), one of 11 members of Congress who had challenged the law, had scheduled a news conference to proclaim victory.

House Budget Committee Chairman William H. Gray III (D-Pa.) was in Philadelphia, so his office made arrangements to contact him by car telephone so that he could make a statement and respond to inquiries.

At the Supreme Court, reporters and spectators jammed the ornate chamber where the court announces its decisions, falling silent as justices entered for the historic moment.

Instead of being witnesses to history, however, the spectators heard the justices opine on the subjects of federal Indian law preemption and tortious conduct on the high seas. They then admitted a group of New Jersey lawyers to the Supreme Court bar and got up and left.

There was no Gramm-Rudman-Hollings ruling.

The crowd had been turned out by an ABC News story Sunday night saying the court would strike down a key part of the law yesterday. The ABC scoop went out over the wires and was reported in major newspapers yesterday.

While there was some speculation that the court delayed the decision at the last moment in response to the news report, informed sources said a ruling in the case had not been scheduled.

The absence of a ruling did not stop some Washington officials, having geared themselves for action, from commenting on its implications anyway.

Budget Director James C. Miller III, for example, declared at a news conference that if the court were to strike down the key provision of the law, President Reagan would comply with the budget reduction targets.

Senate Majority Leader Robert J. Dole (R-Kan.) said the reported ruling would spark a battle in Congress.

The White House, emphasizing that no statement would be made without seeing a ruling, reminded reporters that if the news report was accurate, it would be a vindication of the administration's views.

ABC News correspondent Tim O'Brien reported Sunday night that the justices would strike down, 7 to 2, the automatic trigger provision of the budget-balancing law. He said Chief Justice Warren E. Burger was writing the opinion for the majority, with an important concurring opinion by Justice Sandra Day O'Connor.

O'Brien, in the original broadcast, noted that the highly secret court had, "on at least two occasions that I am aware of, when there were leaks like this, they withheld the decisions until a later date, although the outcome of each ruling was not affected."

O'Brien told reporters yesterday that "we are confident the story is correct and the Supreme Court will announce the decision shortly."

Leaks, though highly unusual, do occur, either to the news media or to interested parties. One of the most famous was the Time magazine story in 1973 outlining the Roe v. Wade opinion legalizing abortion a day before it was announced.

O'Brien in 1979 reported four rulings before they were issued. The court investigated and later transferred an employe believed to have been the source of the leaks. Sunday's report was the first "leak" reported by O'Brien since then.

The court had tentatively scheduled up to three rulings yesterday but issued only two, fueling speculation that the justices, reacting to the ABC story, had pulled back their decision.

Informed sources yesterday insisted that work on the Gramm-Rudman-Hollings opinion had not been completed, that final votes had not been taken and that that opinion was not the one pulled back.

The court's press officer, Toni House, said it was not unusual for the court to issue fewer opinions than scheduled.

Comment on the decision began Sunday night. Sen. Phil Gramm (D-Tex.), co-sponsor of the legislation, preempted everyone by appearing on the initial ABC broadcast. Gramm said the law, which requires automatic, across-the-board cuts if deficit targets are not met, retained a fallback mechanism whereby Congress could pass a joint resolution to meet the targets. Gramm told reporters yesterday that "whatever the problems are, I believe we can fix them."

Vincent Lovoi, an aide to Synar, said his office had invited the other plaintiffs to the news conference, which later was canceled. All in all, Lovoi said, "It was very embarrassing."

Many observers had predicted that the Supreme Court would affirm a ruling last February by a special three-judge panel striking down the automatic trigger mechanism as violating constitutional separation-of-powers principles. In addition, Burger was expected to write the opinion.

House general counsel Steven Ross said yesterday that rumors had circulated last week that the opinion would be coming down yesterday, although he had not heard any details.

Ross said he had investigated the source of some of the rumors only to find out he might have been that source. "I had been saying that I thought the court, for various reasons would act in mid-June." The Capitol Hill rumors, he said, could be traced to "common-sense speculation."

O'Brien's report, Ross said, contained substantially more detailed information.