A Senate opponent of the Equal Rights Amendment threatened yesterday to filibuster a resolution to extend the deadline for states' ratification of the amendment if it reaches the Senate floor.

Sen. Jake Garn (R-Utah) said unless the resolution is amended to allow states to rescind earlier ratification of the ERA he will "support those efforts under the Senate rules, which will provide for a thorough debate of the many issues surrounding extension."

The seven-year extension sought by Sen. Birch Bayh (D-Ind.) would give unfair advantage to supporters of the ERA, unless the 35 states that have voted to ratify if have a chance to retract their approval, Garn said at a hearing on the resolution before Bayh's Judiciary subcommittee on the Constitution.

"It's very much like a basketball game where the losing team says let's extend it for another half, but only they can score points," Garn said.

Bayh, who several times had to reprimand the audience of more than 200 ERA supporters and opponents for applauding when a hearing witness favored their side in the issue, said the extension is vital because there should be "no time limit placed on the pursuit of equality and justice in America."

Sen. Muriel Humphrey (D-Minn.), who led the witness list, said the absence of women in Congress is only one example of the need for the constitutional amendment granting full equality to women. Humphrey is one of two women currently serving in the Senate. She quoted her late husband Hubert's position that ERA is needed to "allow women to take their share of responsibility for the successes and failures of the future."

Equal rights for women are "just as basic" as voting rights under the intentions of constitutional framers, Sen. Edward W. Brooke (R-Mass.) told the subcommittee.

There is a possibility that the resolution will not be voted on by the Bayh subcommittee, according to some Senate staff members. They say the measure is deadlocked with three members for it and three against.

If the resolution fails to be reported out of the full Judiciary Committee, the Senate may still get a chance to consider a measure approved by the House Judiciary Committee last week. That bill would extend the ratification period beyond the March 1979 deadline by three years and three months.

However, Sen. Howard M. Matzenbaum (D-Ohio), a supporter of the extension proposal, said he would seek to move the resolution to the full committee without approval of the subcommittee.