WITH THE SIGNING this evening of a new Panama Canal treaty, what bids to be a memorable struggle over its approval in Congress, apparently in the House as well as the Senate, gets formally under way. It is likely to be a bitter battle: That much is ensured by the passionate certainty with which many Americans hold belief - the quite mistaken belief - that the canal is "ours" and should not be "given away." Then, too, it will be a diplomatically important battle: If Cater administration proves incapable of swinging a treaty that all the Latin nations, and a great many other nations besides, feel is a good and just instrument correcting an indefensible historical anomaly, then American diplomacy will have suffered a heavy blow - and just in the hemisphere alone. The Panama Canal may not seem to Americans to embody the most critical global question, but it stands, like a toll gate, before all the others.

But most of all, the treaty battle, more than any other single episode on the horizon, is likely to shape the whole Carter presidency. Having consummated the complex diplomatic negotiations begun by his predecessors, the President must now conduct what will unquestionably be the tougher and more telling political negotiations with the various congressional opponents of the treaty. This latter exercise will necessarily be conducted in a blinding spotlight. Enough attention has been focused on the treaty, and it is of a certain comprehensible size, to ensure that virtually every citizen and every interested foreign observer will follow how the President deals with it. If he wins, then the country and the world will know that Jimmy Carter is a serious figure, one willing and able to engage the most difficult questions. If he loses, or even if he backs off, people everywhere will conclude that the United States has another weak and in effective president, one not up to the high challenges that international responsibilities put to the United States. Just why other nations would then wish to attempt politically risky negotiations with us - on arms control, the Mideast and so on - is not at all clear.

Some voices now counsel caution. The treaty is, they suggest, too volatile, too unpopular, too emotional, too risky to tackle now - perhaps too dangerous to tackle at all. We find such counsel dangerous itself. A backdown (no matter that it would be called a tactical postponement) would cost the President the momentum that has been gathering in the months of negotiation, would dispirit the foreign and domestic supporters of the new undertaking and would raise the most troubling questions about the President's seriousness. In the interim, the administration would find it hard if not impossible to do more diplomatically than tread water. In the vacuum created by his own hesitation, the notion would likely flourish that the best way to handle the country's foreign problems is not by the reasonable and mutually acceptable adjustment of conflicting interests but by the unilateral assertion of American power. A surer recipe for international disaster is nowhere available.

The imperative need is, then, for action. The President must mobilize all the resources at his command. Fortunately, these are considerable. He has good treaty, one that the Joint Chiefs and responsible conservatives and business interests and liberal internationalists all agree is decidedly superior to the status quo or to any alternative so far identified. He has a legislature dominated by members of his own party. And he has the incentive of knowing that if he flags, not just the treaty and much of American foreign policy but a very substantial part of his whole presidency will have gone down the drain.