PRESIDENT REAGAN has swung aboard the arms and arms control recommendations of his MX commission. This was expected, and it means that quite soon the element that the commission and the president regard as central will be put to a test in Congress. That element is the MX missile, and the question is whether Congress will buy the argument that MX is essential to strengthen deterrence and to stiffen the American arms control hand.

Many in Congress are still caught up in chiding the president for his latter-day discovery that the "window of vulnerability" is no longer open as wide as it was when Mr. Reagan campaigned on a promise to close it with an invulnerable MX. At a certain point, however, perhaps it will be possible for MX critics to take Mr. Reagan's cue of reconsideration and to ask themselves whether their old antipathy to MX still makes sense.

For there is something "new" about MX beyond Mr. Reagan's concession of its vulnerability. It is now being packaged with a proposed new missile, a small one with a single warhead, whose strategic promise is nothing less than to cancel out the dangerous first-strike scenarios associated with the large 10-warhead MX and the Soviet missiles the MX is meant to match.

It is possible, of course, Congress may decide that even if it does deny the next vital slice of R&D funds for MX, the administration will have no choice but to proceed with "Midgetman." But legislators would still have to ask, we think, whether any desirable arms control goal can be reached if the Soviets are not under the pressure of an active American ICBM program. The small missile, after all, is currently just a glint in the eye. The MX program exists.

It is possible to regard MX as a truly bitter pill but to be prepared to take it. And it is possible to see the point of showing the Russians we are prepared to go forward with MX, in order to accomplish the extended transition from the perils of multi-warhead missiles to the greater stability of small, single-warhead ones.

It is possible, that is, if President Reagan can somehow surmount the tremendous wall of distrust of his nuclear proclivities that exists in Congress. There the feeling is widespread that the commission's balanced recommendations merely mask his true intent to go all the way with MX. Arguments that might make a difference if someone else were president leave Mr. Reagan, still, with a difficult uphill task.