The Equal Rights Amendment suffered a potentially fatal blow yesterday when a federal judge ruled that Congress acted unlawfully when it extended the deadline for ratification three years ago, and that states could rescind their ratifying votes.

The decision on extension, if it survives the coming appeals, is sufficient to kill the ERA. Only 35 states -- three short of the required 38 -- had approved the amendment by 1978, when Congress provided an extra three years for ratification. None has approved ERA since.

Even if a higher court overrules that part of yesterday's decision by U.S. District Judge Marion Callister in Boise, Idaho, his ruling permitting states to reverse prior ratification votes still could be lethal.

Callister said that until the amendment is part of the Constitution, any state can rescind its vote by whatever means it chooses.

Five states have reversed their approval of ERA. If they are subtracted from the total needed for ratification, ERA would be eight states short of approval instead of three states short. The odds for ratification -- already slim -- would become insurmountable.

Callister said that once Congress sets a time limit for ratification, "it is not at liberty to change it."

Even if Congress had the power to extend the limit, he said, it could not do so by a simple majority vote, as it did in 1978. It would require the same two-thirds majority in both houses as the Constitution demands for approval of an amendment, he said.

The Equal Rights Amendment proposal says: "Equality of rights under the law shall not be denied or abridged by the United States or by any State on account of sex."

Its supporters say it will eliminate sex discrimination in employment, housing, education and anywhere else it might occur. Its critics say it is unnecessary, potentially disruptive of what they believe to be the appropriate relationship between the sexes, and would eliminate traditional and legal protections enjoyed by women, including protection from the military draft.

ERA supporters will attempt an immediate appeal to the U.S. Supreme Court. The appeal is expected to challenge Callister's ruling as a massive intrusion of judicial authority into the political realm reserved for Congress.

With the extended deadline for ratification set at June 30, 1982, time is already running out for the ERA. The action yesterday, coming as supporters engaged in their most vigorous lobbying campaign yet, was the single most staggering defeat for ERA since it was placed before the states in 1972.

Eleanor Smeal, president of the National Organization for Women, denounced the ruling, attacked the judge as biased and predicted a pro-ERA backlash.

Phyllis Schlafly, leader of the anti-ERA movement, was ecstatic. "It's a great victory for constitutional integrity and fairness," she said. "It means the ERA will never have to be voted on again by any state legislature."

The ruling climaxed a heated 32-month court battle, personalized when ERA backers unsuccessfully sought Callister's removal from the case because he held high office in the Mormon Church, which vigorously opposes the ERA.

Legislators from three states -- Idaho, Arizona and Washington -- first brought the suit. Idaho had rescinded its ratification vote, and representatives from that state asked the court to give effect to that decision. Arizona and Washington legislators challenged the extension, saying that it broke the rules originally set for the states by Congress.

The U.S. government, later joined by the National Organization for Women, provided the defense.

Article V of the Constitution requires a two-thirds vote of both houses of Congress to submit a constitutional amendment to the states for ratification. It says Congress shall determine the "mode of ratification" but does not specifically say anything about time limits for ratification by the states.

Congress approved the ERA by the required two-thirds vote in 1972 and set a seven-year time limit. After the seven years, when supporters sought more time, the extension was approved by a simple majority vote.

Callister said Congress is under no obligation to set any time limit at all on the ratification process. But once it does, he said, "Congress is not at liberty to change it."

Even if Congress were at liberty to extend the deadline, he suggested, it could not do so by a simple majority vote. A two-thirds vote of both houses -- the same margin specified for approval of an amendment -- was necessary, he said.

"Because the extension resolution was enacted by only a simple majority, the extension resolution is an unconstitutional exercise of congressional authority. . . , " Callister's ruling said.

The battle over rescission began immediately after the first few states ratified the amendment. Legislative reversals in Nebraska, Idaho, Tennesee, South Dakota and Kentucky have been in dispute because of procedures followed in those legislatures.

Callister said that states clearly have the authority to change their minds about a constitutional amendment until three-fourths of the states have ratified and the amendment becomes part of the Constitution.

If ratifications were irrevocable, he said, amendments could be added to the Constitution "on technicalities" of voting in each state. "Ratification is linked to that great well-spring of legitimate constitutional power: the will of the people. . . ."

Rescission is "clearly a proper exercise of a state's power . . . especially when it gives a truer picture of local sentiment regarding a proposed amendment."

Moreover, he said, states have "complete discretion" in the legislative procedure they use to ratify or rescind.

A major question in the case was whether a federal judge has the authority under the Constitution to rule on Congress' amendment procedures. ERA supporters argued that the issues were "political questions" beyond judicial scope. That is the same argument made often by conservatives opposing previous "activist" rulings by the U.S. Supreme Court.

Callister said that the amendment power was split between the Congress and the states. The Constitution "did not intend" that either Congress or the states should be its "final arbiters," he said. Rather, "it seems more logical that the courts, as a neutral third party and having the responsibility of 'guardian of the Constitution' decide these questions."

Under federal law, a decision declaring congressional action unconstitutional is directly appealable to the Supreme Court. But ERA supporters need a decision as quickly as possible because the congressional deadline for ratification is just six months away.

The timing is further complicated by the fact that state legislatures may not be in session when and if the Supreme Court rules.