For a measure of the popular confidence in Jimmy Carter's stewardship of national security, you need look no further than the Democratic Party Platfrom Committee's vote this week on the MX missile.

Even as the president was moving through Europe trying to reassert a personal and American leadership role, his fellow Democrats, including some of his delegates, almost succeeded in carrying a vote of no confidence in the program that is the strategic center-piece of his attempt to contain Soviet power.

Only by an appeal to the icon of national security -- "I cannot imagine the Democratic Party would repudiate the president on such an important, fundamental strategic decision," White House aide David Aaron declared -- were the Carter forces able to eke out a narrow symbolic victory.

Even so, the Kennedy forces intend to take MX to the Democratic convention floor. The MX's future in Congress, morever, is still far from assured.

It dismays the administration to keep drawing challenges to the missile it decided to build in order to neutralize the possibility that in this decade the United States' land-based missile force will become at least theoretically vulnerable to a Soviet first strike. That threat is one that any prudent strategist would want to meet in some way. Meeting it with the MX has been regarded as crucial to the administration's now-suspended hopes for ratification of SALT II, and to its claim of providing competent leadership.

Yet the adminsitration is to blame, I think, for much of its own discomfort. From the start, it treated the issue as more or less routine, failing to grasp that special treatment was required to blunt the emotional impact on the public of its becoming aware that the "best" American weapon, one located deep in the heartland and hidden deep in the ground, might not remain secure. It has also had more than the usual difficulty in getting its act together -- only the other week, for instance, it was forced into an unseemly switch from "race-track" to "linear" basing.

The real and steady growth of Soviet missile programs is what created the threat that the MX is designed to ease. Soviet land-based missiles are not -- yet -- similarly threatened. Those at the Democratic platform debate and elsewhere who would dismiss not only the missile but the threat forfeit a serious hearing.

Yet I would go back to what I consider the single most damaging blow to Carter's whole foreign policy: his early decision to forgo wrapping up the SALT deal that Gerald Ford had left ready behind him and instead to offer a big bold new package of his own. In the time and confidence lost by that gesture, Soviet programs gained a momentum that has scarcely been showed since.

MX is, in one aspect, one of those abstract strategic questions that comes down to a duel of hypotheses at close intellectual quarters. But if you had to pick one such question on which the country ought to chew hard, MX is the right one. It confronts a genuine and central strategic dilemma -- the effectiveness of the American nuclear deterrent. Its great cost and its need for vast amounts of real estate and other resources for deployment give it extra political handles easy for a broad public to grab.

This is not to say the public is or should be intimidated by strategic abstractions. In the case of the MX -- and this not the first such case -- the specialists' arguments have been fairly popularized. That is what made it possible for lay delegates to make an honest case against the MX in the platform debate. They argued that the MX would provide a gratuitous American first-strike capability and provoke a matching Soviet program, that in the absence of SALT II's limits on Soviet warheads MX could be overwhelmed, that MX would cost some tens of billions of dollars more than the alternatives and that its deployment would despoil vast reaches of the West.

These are worthly arguments. At the least, the advantages of MX are not so apparent that they wash out the need for further exploring such alternatives as making the existing Minuteman missiles mobile on land or putting them out to sea.

The election of Ronald Reagan, an arms-minded believer in the MX, or John Anderson, an arms control-minded doubter, would alter the political context of the MX debate. That's fine. Any decision on coping with Minuteman vulnerability should flow from an overall strategic/political concept. Jimmy Carter's inability to formulate one explains precisely why he is ending a term in the White House with neither SALT nor MX to show for it.