Comedian Johnny Carson is now ridiculing President Reagan's plan for deploying the MX missile, just as another famous entertainer made fun of President Carter's race-track basing scheme for the missile in 1980.

The other entertainer was Ronald Reagan, then a candidate for president.

Only in America could a nuclear weapon powerful enough to blow up the world become the stuff of a Johnny Carson monologue on NBC television's "Tonight" show.

How this came to be is a story of presidents, defense secretaries, generals, admirals, senators and representatives being flung off as they tried to ride that bucking horse of the arms race, MX -- for missile experimental. The missile still is going its own way without anybody knowing where it will end up.

"The fellows who are supposed to be the highest military tribunal in our country," Carson said Thursday night in talking about the majority of the Joint Chiefs of Staff, "recommended against the 'Dense Pack' theory of putting those missiles in silos. Unfortunately, they were overruled by the star of 'Hellcats of the Navy.' "

The official record of the MX contains many more ironies, such as these:

* The MX has 10 nuclear warheads that could be used in a first strike but was born when civilian policy leaders were telling Congress that the United States was not interested in developing first-strike weapons. The Pentagon's justification for building the MX has been schizoid ever since.

* The Air Force now admits that it wants the MX primarily to increase its capability to blow up Soviet missile silos even though those silos would be empty if the United States stuck to its proclaimed nuclear policy of holding its fire until fired upon.

* The Joint Chiefs under President Carter recommended rotating MXs among vertical silos spread far apart but said they could agree to trucking them over gravel roads from one cement garage to another to make it easier for the Soviets to keep track of the missiles for arms-control verification.

Last month, the chiefs told Reagan they had split, 3 to 2, over the proposal to use vertical silos bunched closely on the Wyoming prairie in the Dense Pack formation.

The Air Force told Congress that one good thing about Dense Pack is that any Soviet warheads exploding on the Wyoming prairie would kick up boulders, "some of them larger than a Volkswagen," making it difficult for the attacker to "find a window in which he can confidently attack."

The Pentagon has elaborate explanations for these seeming contradictions. But after hearing them in a closed session of the House Appropriations subcommittee on defense, Rep. Jack Edwards (Ala.) ranking Republican and ally of Reagan on MX, lamented:

"I am supposed to be one of the hawks on the committee, I guess, but I swear the more I sit here and listen to this, the more I wonder what in the world we are up to."

One must go back to President Kennedy's "missile gap" campaign of 1960 to try to explain how the United States is on the verge of producing its most dangerous missile without knowing where to put it.

Candidate Kennedy accused President Eisenhower 22 years ago of allowing the Soviets to get ahead of the United States with ICBMs. His defense secretary, Robert S. McNamara, admitted after taking office in 1961 that he could find no such missile gap. Then McNamara made a decision that planted the seeds for the MX, although that was not his intention.

McNamara approved pushing ahead with technology that would enable one missile to carry as many as 14 different nuclear bombs and hurl each to a different target. This was MIRV, for multiple, independently targetable reentry vehicles. The United States perfected MIRV years before the Soviet Union, but the Soviets went all out to close the sudden missile gap.

"The MIRV capability," John S. Foster Jr., then Pentagon research director, told Congress in 1970, "is a low-cost and efficient way to maintain our rational policy of deterrence without increasing the number of our expensive missiles. The use of mulitiple warheads can provide effective penetration of Soviet anti-ballistic-missile defense systems and less overkill through a more efficient use of the booster payload."

In other words, the Pentagon's civilian research director was advertising MIRV as a bargain-basement way of covering more Soviet targets with the existing U.S. missile force while providing a way to overwhelm any Soviet defense by launching more warheads than it could handle.

On Nov. 5, 1970, Defense Secretary Melvin R. Laird tried to assure a concerned Sen. Edward W. Brooke (R-Mass.) that MIRV was not being developed to blow up Soviet missiles in their underground silos by bracketing them with nuclear warheads launched by a single missile. Such plans would make MIRV a "counterforce" weapon designed to catch Soviet missiles in their silos in a surprise attack.

"We have not developed, and are not seeking to develop a weapon system having, or which could reasonably be construed as having, a first-strike potential," Laird wrote Brooke.

Air Force generals were saying something different among themselves and occasionally in public. Their Minuteman III missile with its MIRV warhead, then Air Force chief John D. Ryan proudly proclaimed to the Air Force Association on Sept. 22, 1970, "will be our best means of destroying time-urgent targets like the long-range weapons of the enemy."

Just one year later, the Air Force Strategic Air Command, which is in charge of the nation's nuclear missiles and long-range bombers, was sketching an ever-better silo buster than Minuteman. It came to be called the MX.

Soviet generals, watching these developments and allegedly stealing as much MIRV technology as possible, developed multiple warheads for their own more powerful but less accurate land missiles. Laird dramatized the action-reaction phenomenon of the arms race by publicly accusing the Soviets of arming their SS9 blockbuster missile for first-strike attack against U.S. missile silos.

James R. Schlesinger, defense secretary from 1973-75, went Laird one better by declaring in his 1974 posture statement that the heavier lifting capability of Soviet rockets could hurl so many MIRV warheads at U.S. missile silos that "they would then possess a major one-sided counterforce capability against the the United States ICBM force.

"This is impermissible from our point of view. There must be essential equivalence between the strategic forces of the United States and the U.S.S.R."

In saying that publicly, Schlesinger had legitimized development of MX as a silo-busting weapon. His thesis was that the United States must have missiles that could do as much damage to Soviet ICBMs as theirs could do. How, when and where to deploy the new blockbuster missile was left then, as now, for later.

Schlesinger accelerated MX development, which has proceeded to the point that the Air Force this year is asking $988 million to produce the first five missiles, an amount the House deleted last week. Reagan is urging the Senate to restore it this week.

Lost in the current MX debate is the fact that the U.S. decision to give its missiles MIRV capacity in the 1960s opened the "window of vulnerability" in the 1980s. MIRV warheads, first justified in part to penetrate missile defenses, still were pursued vigorously by the United States and Soviet Union after they agreed not to deploy such defenses.

Today, the Pentagon claims, the U.S. window of vulnerability is wide open because the Soviets have deployed so many MIRV warheads on large missiles that 95 percent of the U.S. force of 1,000 Minuteman and 51 Titan intercontinental ballistic missiles could be destroyed in a surprise strike.

To close this window, Reagan contends, as did his two predecessors, that the new MX land missile must be deployed in such a way that the Soviets could not destroy it without firing an unacceptably larger percentage of their MIRV warheads. A survivable force of 100 MX missiles, the president contends, would continue to make nuclear war look like such a losing proposition to the Kremlin that its leaders would not dare to push the button.

Critics contend that both sides have deployed so many accurate warheads that their land missiles could not be saved except by firing them at the first sign of attack. This situation raises the specter of world incineration through false alarm, one of the reasons behind demands that first-strike weapons be scrapped.

The Joint Chiefs of Staff would welcome that. They are complaining privately that so much money will go for nuclear strategic weapons under Reagan's existing rearmament plan that conventional forces will suffer. But they have not recommended giving up on trying to make land missiles invulnerable and putting more nuclear firepower instead in airplanes or on ships or submarines.

The chiefs of the Army, Navy, Air Force and Marine Corps have often conflicting responsibilities. Each is supposed to run his service day by day but change into what the Pentagon calls "the purple suit" when sitting down as part of an advisory body to the president and secretary of defense.

As military officers, they must support the commander-in-chief's decision or resign from the service. Thus, in saying the chiefs agreed to support his Dense Pack choice after they split 3 to 2 against it, Reagan is documenting the obvious in the real life of the military.

Reagan, who as a candidate made fun of Carter's "fantastic plan to take thousands and thousands of square miles out in the western states" to put the MX missiles on a race track, today sounds less convinced that Dense Pack is much better than Carter's proposal.

So many lawmakers feel the same way about Dense Pack that the MX remains the missile all dressed up with rhetoric but with no place to go.