The MX supermissile, a critical ingredient in the package of inducements the White House has assembled to promote SALT II, appears to face more serious opposition in the Senate than previously realized.

In an unpublicized vote recently, the Senate Appropriations Committee surprised its own members by voting just 13 to 8 against a motion to delete all funds for the MX. And it is relatively easy to find Senate members and staff assistants -- even some who favor the MX -- who predict the elaborate missile system will never be built.

The MX has had an ironic history. The Carter admnistration has backed it avidly to satisfy senators who feel the country cannot afford the strategic arms limitation treaty without a new missile system. But many officials inside the administration,including Adm. Stansfield Turner, the director of central intelligence, have opposed the MX plan.

Today pro-SALT senators who have misgivings about the MX are supporting it for the sake of the treaty. One example, apparently, was Birch Bayh (D-Ind.).who supported the MX in the Appropriations Committee despite some doubt about its efficacy.

The vote in the committee came on a motion by Mark Hatfield (R-Ore.) to delete all funds ( $670 million) for the MX from next year's defense budget and redirect $20 million to study a submarine-based alternative plan. Hatfield was amazed to get eight votes for his proposal in the committee and just 13 opponents (a later poll of the full membership made the vote 18 to 9.)

One surprising Hatfield ally was Henry Bellmon (R-Okla.), who predicted that the MX will eventually cost $120 billion. (The Pentagon says it will cost $33 billion; the Congressional Budget Office predicts a figure nearer $60 billion.) "I am contrary to a multibillion contraption that probably will never be finished," Bellmon said.

Another surprise was the vote of Ted Stevens (R-Alaska), the minority whip and a SALT opponent. Stevens even criticized Bellmon for wanting to preserve and redirect $20 million of the MX money. "I would totally zero it," Stevens said, "and it would be the first time I voted like this on a defense position."

Hatfield will reintroduce his motion on the Senate floor this week or next. His aides predict it will fail this time, in part at least because many SALT supporters will vote against it for the sake of the treaty. But eventually Hatfield and other senators expect the MX to be killed in Congress.

Sen. Daniel Patrick Moynihan (D-N.Y.), a relatively hard-liner on defense issues, predict flatly that the MX will never be built. "I think that this MX is the kind of technological exotica that large organizations can get caught up in, and it's only when everyone else sees it that they look up and say, "that's crazy.'"

It may be typical of the irony beclouding MX that despite this view, Moynihan wants to stick with Senate colleagues who are seeking a stronger American defense, he explained in an interview.

A combination of practical and political factors underlies the predictions that MX will not be built. The practical problems posed by this most complex of all existing weapons systems are staggering.

Simply building it will be a gargantuan project, comparable to the biggest public works undertakings in American history. As proposed by the Carter administration, there will be 200 MX missiles, each of them attached to its own immense tranporter-launcher, a truck-like device.

Each missile will carry 10 independently targetable nuclear warheads, and each will be based on its own paved "racetrack" in the desert in Utah or Nevada.

The idea of the MX is to fool Soviet targeters so they can never know exactly where the MX missiles are hidden. This is intended to preserve the invulnerability of American land-based missiles in an era when the growing number of Soviet warheads will soon give the Soviets a theoretical opportunity to strike simultaneously at all of America's existing, fixed-silo ICBMs.

This concern over the potential and theoretical vulnerability of the land-based missiles has alarmed many senators, including the majority leader, Robert C. Byrd (D-W. Va.). before endorsing SALT II, Byrd sought and received written assurance from President Carter that the MX would indeed be built.

But presidential assurances cannot overcome all the obstacles that lie in the path of the MX. For example, the proposed project will have to satisfy the conditions of 33 different federal laws, ranging from the endangered species act to various clean water and air statutes. Almost every one of these laws could provoke lawsuits from arms control advocates or environmental activists, and representatives of both groups have already begun planning such legal action.

Water for MX is a particularly contentious subject. Just yesterday the governor of Utah, Scott Matherson, warned that he would withdraw his tentative endorsement of putting MXs in Utah if the federal government sought to avoid state controls on the water supply. Matheson also requested $2 billion in federal aid to Utah and Nevada to cope with the social and economic repercussions of the MX project.

The MX will require congressional approval for the transfer of lands now administered by the federal Bureau of Land Management to the Air Force. Two of the most liberal committees on Capitol Hill -- Senate Energy and House Interior -- would have to give their assent.

To prevail on MX the Pentagon will have to persuade both Senate and House to go into this enormously expensive undertaking -- involving not only 200 racetracks and 4,600 shelters, but an elaborate railroad, huge barriers and other gimmicks to convince the Soviets that there is only one rocket on every racetrack.

Persuading Congress to appropriate these tens of billions might be easier if MX was guaranteed to succeed. But even administration officiials acknowledge that as now designed, the scheme can work only if the limits contained in SALLT II stay in effect. If they lapse, the Soviets could relatively easily add new warheads beyound the number SALT II permits to neutralize the MX or force the United States to expand it greatly.

In the end, some MX critics predict, the size, cost and Rube Goldberg complexity of the MS plan will bring it down, perhaps leaving the United States with no new weapons system to compensate for recent Soviet advances.