THAT WAS NO trivial defeat President Reagan's MX proposal suffered in the House. With Democrats voting against him 195 to 38, he needed at least 173 of that day's available 188 Republican votes. He got only 138: 50 members of his own party repudiated him on an issue he had defined as vital to his strategic posture and negotiating strategy alike. The outcome represented a first major defeat in an area, national security, in which Mr. Reagan has heretofore ridden high. The House majority, the president said, had been "sleepwalking."

Mr. Reagan's MX proposal was in trouble from the start. Defense conservatives felt that the latest basing mode, Dense Pack, raised as many questions as it answered. Nuclear freezers and other arms controllers were, naturally, opposed. Budget cutters saw that, although only $1 billion in production money would be saved now, many more billions might be saved later. Others felt that defense is too important, and the available resources too short, to start down any major new road without a better argument in favor of what the administration proposed. In fact, the administration has never satisfactorily addressed the central question of whether the country should retain its old emphasis on land- based missiles, as unavoidably vulnerable as the rush of technology makes them appear to be.

The president vows he will take his MX case to the country and reverse the House vote. No one should underestimate him. We wonder, however, precisely what calculation of votes and stakes makes him ready to risk more of his political capital on this project. We also think the administration does its own bargaining position with the Soviets considerable harm in poor-mouthing our nuclear condition without MX. Secretary of Defense Weinberger protests that "we are disarming unilaterally." Would it not be wiser to emphasize -- not least to Moscow -- the other Congress-proof weapons that the United States still has available both for deployment and for bargaining purposes? The MX, after all, is only one system of many that the administration either has in hand or is virtually certain to build. By claiming that it is essential to showing the Russians we are strong and mean to stay strong, the system's proponents send a wrong and false message to the U.S.S.R.