IT HARDLY MATTERS whether Sunday's demonstration for the Equal Rights Amendment totaled around 50,000, as some police estimated, or nearly 100,000 as the Park Police guessed. Even the lower figure is an impressive turnout on a sweltering Sunday in July. Moreover, the marching, chanting throng on the mall got ample publicity, partly because - by accident or design - its organizers picked a day when little else was going on. The only question is whether all that impressive effort and energy might not have been better expended in pursuit of a different political objective and in pursuance of a sounder political strategy.

The immediate purpose of the march, and of an intensive lobbying effort this week, is to spur Congress to allow another seven years for ratification of the ERA by the states. The extension has become an obsession for many pro-ERA groups as the prospects for approval by three more states before next March 22 have grown more and more dim, especially since the narrow defeat in the Illinois House Last month.

In our view, however, ERA advocates would do better to concentrate on moblizing their friends around the country - not for marches, but for sharply focused political organizing, starting with this fall's congressional and state campaigns. We say this partly because we are opposed to the kind of rules-changing involved in extending the congressional deadline. But it also seems clear that grass-roots efforts and attitudes will determine ERA's future in any case. Most public-opinion polls, including a new one by CBS News and The New York Times, show substantial nationwide support for the cause of equal rights. But the amendment is not likely to be adopted, in seven months or seven years, unless the sentiment shown in such polls becomes a political force with unmistakable impact at the polls on election day.

This is true no matter what the timetable may be. Winning ratification by March 22 will require some dramatic political educating in areas that are quite unpromising. But the job may be even harder, as well as longer, if the extension is approved in some form - because ERA supporters then will have to gain three more victories, and also stave off a flock of efforts to rescind earlier approvals in various states. And if no extension is granted, the whole effort will have to start again from scratch.

In any case, then, the crucial message that needs to be sent is the one that was chanted so often on Sunday: "ERA won't go away." And that will have to carry a very pointed meaning - not just that a large collection of groups are going to keep fighting for the amendment, but that many citizens are going to remember how various officeholders have performed, and vote accordingly.*