PITY THE HISTORIANS who, looking back at the climactic moment of the Senate's consideration of the Panama Canal treaties, find themselves peering at Dennis DeConcini. How was it, they may ask, that the treaties, with their immense diplomatic and political freight, came to hinge on the ill-informed whims of a 40-year-old freshman senator of no previous renown, of no known international awareness, of little experience of any kind beyond minor administrative posts in Arizona? How could a supposedly responsible U.S. Senate in effect delegate its power, in a matter as sweeping as this, to a lightweight whom serious senators should regard as an institutional embarrassment? How was it that this obscure legislator came to be cultivated by the president - they may meet again today - in a manner itself evidence of the humbling of the highest office of the land?

The bargaining over the second treaty has been intense. The administration has had to deal with Mr. DeConcini to ease the effects of the reservation he was able to attach to the first treaty and to head off a similar desecration to the second. The "DeConcini reservation" would give the United States a unilateral right to intervene in Panama, if it perceived a threat to the canal. The certainty and legitimacy of Panama's objections to such open trampling on its sovereignty compel the administration to keep trying to do what it can, in language added to the second treaty, to render the package acceptable to Panama.

But there is a limit to what even as hard pressed a president as Jimmy Carter ought to accept in bargaining with the likes of Dennis DeConcini. He blundered sorely in failing to anticipate the explosive Panamanian reaction to his acceptance of the DeConcini reservation the first time around. But he cannot in honor take back to Panama a package that he and everyone else knows is absolutely offensive to virtually every segment of Panamanian society, a package, moreover, joltingly different from the two texts that Panama ratified by plebiscite last year.

Whether Mr. DeConcini can comprehend the dimensions of the mischief he has wrought is not altogether clear from his own observations. There are some signs he realizes that he appears, to some constitutents as well as to fellow senators, ludicrous and irresponsible and hopelessly out of his depth, but they are not conclusive. In any event, it is up to Mr. Carter to impress on him that if he persists in his ways, the wreckage will be on his hands and on the hands of those other Senators so cynically playing his game.