IT IS ALMOST a foregone conclusion that Congress will approve today a 39-month extension of the period during which states can ratify the Equal Rights Amendment. The House passed the necessary resolution in August, and the votes by which amendments to the resolution have been defeated in the Senate suggest that the Senate will pass it easily today. Although we have expressed reservations in the past about the wisdom of this extension, we join now with those who are urging the Senate to approve it overwhemingly.

Our reservations about the ERA extension never included the basic argument made against it on Capitol Hill - that Congress lacks power to extend. The Constitution gives Congress the principal role in the amending process, and it can do almost anything it wants in terms of time limits and tabulating state ratifications. Our concerns were with the fairness and the wisdom - in terms of precedents as well as politics - of changing a time limit once Congress had established it. We believed such a change might diminish support for the ERA as well as destroy the useful idea that amendments should represent the prevailing national consensus during a predetermined period of time.

Proponents of the extension have made a strong case that the ERA ought to be regarded differently from most other constitutional amendments. Unlike most of the others proposed in the last century, which dealt with the structure or powers of government, ERA is directed at a social, economic and moral issues: the status of women in society. Unlike the others, it has been subjected to an unprecendented outpouring of deceitful or untrue propoganda. And, most important, unlike the others, the ERA has never been given fair consideration in several state legislatures where internal politics or procedural games have taken precedence over honest consideration.

A majority of the members of Congress - and the president - have been persuaded by these arguments that the unfairness already done to the ERA by its opponents more than offsets the potential unfairness of a ratification extension. Insofar as that is a judgement on the tactics employed in the past by some of those opposed to the ERA, we agree; they have been not been willing to let the ERA stand or fall on its own merits.

A year ago, when this extension was proposed, it seemed possible that it would not be needed. There was hope then that in certain state legislatures - most notably Illinois, Florida and Florida and Virginia - the issue would be handled fairly this year. But it was not. This, more than anything else, negates any concerns that an extension is either unfair or politically unwise.

Because of what has happened in the legislatures, it is not unreasonable for Congress to decide that proponents of the ERA should have an additional 39 months to persuade a handful of states that it should be ratified. The time period is short - less than half of what was sought a year ago - and the issue is very much alive. More than two-thirds of the states have already said they want this amendment in the Constitution. The extension is, in reality, a concession by Congress that it misjudged six years ago the time needed for a controversial amendment to receive fair consideration in 50 legislatures.

We would rest easier with this extension if Congress had adopted the view that during those 39 months states could also rescind their prior ratifications. That would have reduced the drawbacks of the precedent that is being created. But that view has been rejected on both sides of Capitol Hill. One can hope, at least, that the value of this extension as a precedent will be discounted in the future because of the peculiar nature of the factors that made it necessary now.