The Justice Department advised Congress yesterday that it has the right to extend the deadline for ratifying the faltering Equal Rights Amendment.

The department further said that states which had already voted for the amendment could not reject it later.

In a legal opinion sent to the White House and the House Subcommittee on Constitutional Rights, the Justice Department states the Congress has the sole power - by a majority vote of both houses - to extend the seven-year time period for ratification of the ERA. Rep. Elizabeth Holtzman (D-N.Y.) has introduced a bill calling for extension of the deadline to 1986.

Thirty-eight states must vote to ratify the ERA by March, 1979, for passage of the amendment unless the deadline is extended. Of the 35 who have passed the ERA, three - Idaho, Nebraska and Tennessee - have sine voted to rescind.

John M. Harmon, assistant attorney general for the office of legal counsel, argued that rescinding is invalid. Congress has consistently interpreted the Constititution as giving the states the "power to ratify a proposed amendment, but not the power to reject. It is my view that a state is powerless to rescind" a previous ratification, he said.

Harmon was the opening witness at the subcommittee hearings on the constitutionality of extending the ratification period. Although Laurence Tribe, professor of law at Harvard Law School, agreed with Harmon that Congress had the sole right to extend the ratification deadline, he argued that the power to rescind is by no means a closed question.

In Tribe's opinion, the ultimate decision on whether or not to count those states which now have or might rescind in the future rests with the Congress sitting at the time 38 states have passed the amendment.

When Congress passed the ERA in 1972, it placed the ratification deadline of 7 years in the proposing resolution rather than in the actual text of the amendment. Had the time limit been incorporated in the amendment, Harmon testified, it is unlikely is could be extended.