AS SO OFTEN happens, the choice before the House in considering the MX missile was posed in terms that many congressmen, for different reasons, wished to revise or escape. But the House could not avoid the issue in the form -- up or down, no conditions attached -- that actually lay before it, and it did not. It made a close but sound decision in boosting the second batch of 21 MX missiles over their fourth and last parliamentary hurdle of this session. Cut now to Geneva.

There is widespread agreement that on its merits -- and how quickly the full complement of Congress became expert, or easy-talking anyway, on the merits -- the MX would have crashed. Its fate was bound to pivot on the perceived probable effect of approval or rejection on the arms control talks that resumed in Geneva a few weeks ago. The Reagan administration acknowledged as much by bringing back the chief of its Geneva negotiating team for 11th-hour lobbying, while the secretary of defense toured in Europe.

For many legislators, strong convictions or strong political pressures may have swayed their vote. The issue admitted of different and equally conscientious answers, and certainly it became extremely politicized. But for a swing bloc of legislators, most of them Democrats, the administration's insistence on needing MX to strengthen the president's Geneva hand, or at least to prevent the weakening of his hand, added a painful extra burden. This group favors the idea of effective arms control as a tool of American security but harbors strong misgivings about President Reagan's commitment to it. These legislators had to face the possibility that by voting for the missile they would end up helping him avoid serious negotiations and that in any event they would pay politically for their vote. With no little courage, they took the risk. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, was their leader.

As a practical matter, it will take some time at Geneva for President Reagan to learn whether his MX victory was worth the tremendous struggle he waged to win it and for those who supported him to learn whether they were wise to do so. We think, nonetheless, that a conscientious Congress could not possibly have cut off the president in these early-Geneva, early-Gorbachev circumstances.

There is a residual doubt about President Reagan's approach to arms control -- and a great deal more than a residual doubt about the Kremlin's. But there is also a residual awareness that the position of strength Mr. Reagan has built (with help from his predecessors and, in the MX vote, from some of his political rivals) and the image of strength he has fashioned for himself give the United States some special opportunities now at Geneva.