THE REAGAN administration had, by all accounts, a devil of a time deciding what to do about the MX missile and the B1 bomber. Many combinations were inspected, and the arguments evidently did not subside until the end of a months-long review. And now, as is characteristic in these big weapons programs, the president's final choices are being presented as a national imperative, the product of a logic beyond further cavil.

But wait a minute. The decisions on the MX and B1 are nothing if not eminently arguable. They were arguable before on the inside, and they are arguable now on the outside. Ronald Reagan was able to use his presidential authority to cut short the first debate, but Congress will have to be persuaded in the second. The president in our judgment deserves great credit for eliminating the over-elaborate hide-and-seek concept. But questions immediately arise about the ideas he replaced it with.

What needs to be explored first, in respect to the MX, is the nature of the threat that the MX, however and wherever deployed, is designed to meet. Theoretically, as its adovcates say, the Soviets could overwhelm American land-based missiles, or threaten to, or leave Americans fearing that they might, and thus paralyze our capacity or will to retaliate -- a process whereby they could reap political gain. But to the extent such a scenario seems real -- and that is much a matter of argument -- the MX would not close the perceived current "window of vulnerability" for five-plus years. In the meantime, new technologies will inevitably bring new contingencies and opportunities to mind: cruise missiles -- cheap, hideable, super-accurate -- are already coming on line. In the absence of negotiated limits on the number of Soviet warheads, moreover, how are the Soviets to be kept from covering the MX holes with additional warheads?

We do not say these questions are unanswerable or that answers, to be acceptable, must achieve total buttoned-up, no-loose-strings perfection. But any large new procurement decision must seem on the whole reasonably necessary and sensible. Mr. Reagan still has some explaining to do to show that his MX decision meets this standard.

Some of the same kinds of worries attend his statement on the B1 bomber. Given the great Soviet investment in bomber defense, the availability (and improvability) of cruise missiles, and the R&D being done on a more detection-proff "stealth" bomber, not to speak of budget realities, is it really sound to start down the B1 route? Perhaps it is, but the administration remains under a burden to demonstrate that it has sorted out all these considerations in an orderly way.

Something else needs to be underlined. The administration's procurement plans are not just procurement plans; everyone understands that. They are political plans: they announce a certain attitude toward national security and toward the Soviet Union. So these plans cannot, and are not, being left to the specialists and the strategists. They are being enveloped in the general political debate.

Mr. Reagan is in a position to say he received an electoral mandate to "rearm America" and to achieve "a margin of safety," his term for superiority, over the Soviet Union. He is entitled to represent the MX and B1 as his partial delivery on these promises. But is the United States more secure with a missile that Moscow can target any number of new warheads on, and with a bomber with such questionable chances of making it past the Soviet border? "Signals" are not inconsequential: is the United States going to look stronger several years down the road, when the big costs of these programs hit the budget and start competing with other pressing military needs?

There is a certain tendency in the administration to dismiss doubters as sore losers, softies on the left. But so-called defense conservatives, whose agenda is to strengthen defense, get good defense value for the dollar and maintain steady public support for defense over time, are also concerned. Within their ranks is where we take the center of gravity in the Congress to be. That is where the real defense debate will now begin.