AS THE HOUSE Democratic leaders describe it, the House's mangling of the gas-rationing bill Wednesday was a horrible mistake. Some members got confused, they say, and voted for a crucial amendment without realizing how devastating it was. This week they will be ready to "repent," as Democratic Whip John Brademas (D-Ind.) put it, and approach the bill more sensibly.;

Let us hope they're right. The House certainly behaved badly on Wednesday. It seemed even more shortsighted and parochial than it had back in May, when it rejected President Carter's earlier contingency plan for gas rationing. The issue last week, after all, was not any particular plan; it was Mr. Carter's request for authority to devise a new standby plan and put it into effect if a genuine fuel emergency occurred. That sounded more likely to pass - especially since House Democrats were said to be chagrined by the May debacle and eager to show their support for Mr. Carter's new energy effort.

But the House choked. A majority seemed to oppose giving the president any real emergency authority. At least they insisted on defining an "emergency" so restrictively that the nation's industry and commerce might be largely shut down for a month before the chief executive could ration gasoline.; Then the House voted to give itself and the Senate not one but two chances to veto a new rationing plan: once when the president proposes it, in advance of an emergency, and again before the plan could actually be used.

Granted, no one should relish the prospect of rationing. The current allocation system is bad enough. Rationing involves problems so vast and inescapable that decontrolling prices or imposing a much stiffer tax, or both, are preferable tools for curbing consumption in most circumstances,.

And yet, the country does need some last-dtich contingency plan - and some confidence that it could be invoked promptly if a genuine crisis hit. That's what the House forgot Wednesday. Too many members seemed preoccupied with trying to protect their constituents' everyday gas supplies. But in the sort of emergency Mr. Carter wants to be ready for, a lot of everyday habits and conveniences will have to be given up. No prospects would be pretty. At some point the choice might even be between rationing gasoline and rationing food or heat. The House should contemplate those possibilities - and get serious.