IT WAS UNFORTUNATE that Congress left town without authorizing a stand-by plan for rationing gasoline. But it was hardly surprising. Rationing confronts Congress with an uncomfortable dilemma. It knows that it cannot write rationing rules itself, as President Carter challenged it to do; the collisions of regional and social interests are too sharp and complex. But it also knows that the White House proposals have been uniformly inept, and the legislators resist giving Mr. Carter any very broad latitude to work it out by himself.

That's why the House has been anxiously trying to write certain principles and suggestions into the bill, without quite daring to reduce them to specific requirements. That's also why the House keeps hedging the bill with provisions making it impossible to put rationing into effect except in the direst of national emergencies. The White House resents those conditions, sensing - accurately - that they indicate a lack of confidence in the president's judgment.

There was a time, several years ago, when rationing was a popular idea in Congress. But in the debates last spring over the president's first plan, congressmen and senators began to perceive the enormous regional impacts of little provisions that looked, at first glance, like the merest of technicalities. During the shortages of May and June, they saw the deep and unpredictable effects of even the smaller defects in the federal allocation formulas. Congress has become too sophisticated and experienced in the pitfalls of shortage management to be able to move quickly or easily. From the public's point of view, that's not necessarily a bad thing.

The original Carter proposal would have given a ration book to each car. That means three times as much gasoline to the three-car family as to the one-car family. Is that fair? Since the stamps could be sold legally, is it right to omit people who don't have cars, or even licenses, from this lucrative benefit? Who is to weigh the hardship cases? What about the small business that needs gasoline, but would be bankrupted by buying ration stamps on the open market? Whether they want to or not, great numbers of people in Congress now see what rationing would get the government into.

But that, again, is the dilemma. If some catastrophe - a war, say, in the Persian Gulf - suddenly imposed a drastic drop in the world's oil supply, the country would need rationing fast. By most estimates, preparing the system - printing the coupons, writing the rules, programming the computers - would take at least six months. That's the work that would be done under the stand-by authority. It's worth doing, as a kind of insurance. But the doubts and delays in Congress aren't caused by cowardice, as the Democratic leadership keeps angrily charing. The explanation is that, in the past six months, the members have learned too much for comfort about the things that would go wrong in rationing.