In the face of their latest and perhaps most severe setback, leaders of the equal rights movement yesterday did exactly what they have done after so many other defeats in recent years: They vowed to keep fighting.

They allowed few words of remorse. They claimed over and over that the drive for the Equal Rights Amendment, which began in 1923, is not dead, despite a federal judge's ruling that the deadline for its ratification was unlawfully extended by Congress and that five states were within their rights when they rescinded approval.

"Why in the world should proponents who have fought so very hard for so very long give up now?" said Eleanor Smeal, president of the National Organization for Women. "We estimate that only six or eight people stand in the way of equal rights. Why should we be faint-hearted now?"

The ruling, Smeal said, was "a political decision" made by "one man," U.S. District Judge Marion Callister, whom she described as "an obscure judge" in "the lowest federal court."

She also repeated ERA proponents' contention that as a high-ranking official in the Mormon church, which strongly opposes the amendment, Callister should have excused himself. Mormon officials had no comment yesterday on that criticism and said of Callister's action, "We accept the decision of the courts of the land."

Smeal and other leaders in the women's movement called the ruling "outrageous," "appalling" and "shocking," but not unexpected. They predicted the decision would be overturned.

The proponents insisted they would continue to lobby for approval of the ERA by state legislatures in Florida, Virginia, Oklahoma, North and South Carolina and Illinois.

But Callister's ruling clearly puts the proponents in a box just weeks before the legislatures in a number of states are to meet. Despite a sophisticated lobbying campaign and polls that show 60 percent of the American public favoring the amendment, they have not been able to persuade a single state to ratify the ERA since 1977.

"We knew we had an uphill battle before the decision, but we were really thinking things were looking up," Ruth Hinerfeld, president of the League of Women Voters, said in an interview.

A few minutes earlier, she had told a press conference: "The campaign for equal rights has undeniably suffered a blow . . . . Legislators will seize on this decision as an excuse for inaction."

ERA opponents, meanwhile, hailed Callister's ruling. Phyllis Schlafly, head of Stop ERA, said from her home in Alton, Ill., that the decision was a "great victory."

"It shows that ERA really died on March 22, 1979, at the expiration of the seven-year time limit originally set by Congress," she said. "We threw a big bash in Washington that night."

Kathy Teague, president of Stop ERA Virginia, said it is all but certain that the Virginia legislature won't consider ratification now. "The proponents didn't have the votes to pass it before the decision, and they certainly won't now."

Lulu Meese, chairman of the ERA Ratification Council for Virginia, disagreed strongly with Teague's view of the amendment's chances, saying that ERA backers in the state are "not disheartened" and "expect to ratify the ERA in Virginia."

Senate Democratic Whip Alan Cranston (D-Calif.), a leader in the fight for the extension, agreed with the assessment of ERA opponents, saying that Callister's decision makes ratification "impossible unless something changes."

Sen. Jake Garn (R-Utah), who opposed the extension, praised the ruling, saying: "I said at the time that the extension was unfair and unconstitutional and Judge Callister's ruling upholds that assessment."