At one point in the bizarre hearings on the born-again MX missile, Secretary of Defense Caspar W. Weinberger and Sen. Henry M. Jackson (D-Wash.) engaged in a discussion of the nuclear freeze, which at that moment was being debated in the House of Representatives--and might have passed unanimously had the Senate Armed Services Committee proceedings been piped into the House chamber.

Jackson, who disapproves of it, opined solemnly that the reason for the worldwide freeze craze stems from a "fear of the instability of political institutions."

Actually, it stems from a fear of nuclear weapons. The reason millions of their constituents are badgering the representatives is that they fear that two quite stable political institutions, namely the United States and the Soviet Union, might blow up the world.

Weinberger wisely assured Jackson that the freeze is bad because it freezes our military inferiority.

Weinberger and Secretary of State George P. Shultz--one slight, pale and intense, the other large, ruddy and impassive--were making the hard sell for the new solution to the MX problem, the one dug up by President Reagan's Commission on Strategic Forces. It calls for 100 "Peacekeepers," as the president dubbed them, to be housed in super-hardened existing Minuteman missile silos, a plan flatly rejected by a generation of strategic planners, including Weinberger.

As outlined by Shultz and Weinberger, the new scheme is really an arms-control move of almost irresistible seductiveness--if you count the wrapping, which is an offer to shift, in time, to single-warhead missiles, and to make arms-control counts in terms of warheads instead of launchers.

Without the MX, it's not a bad idea. A number of Democrats have signed on, apparently on the premise that you'll never cure any American president of an obsession about having a land-based missile.

Its basic trouble is the assumptions it makes about the Soviets. No other conclusion can be reached--nor was it by the two Reagan heavyweights--than that the MX will be as vulnerable as ever.

"It's not a perfect basing mode," Weinberger generously admitted, as he had to, because two years ago he said that very same thing.

Even though the Soviets have read the statements about the folly of choosing already targeted sites, they will nonetheless, the administration argues, go along with the gag. They will, it is argued, leap to frightened conclusions about our "national will" and "resolve" and break a leg getting to the bargaining table.

They will also--and Weinberger was emphatic on this point--fall to squandering more of their national treasure on constructing measures to counter a threat that our experts have claimed was flimflam.

Neither Weinberger nor Shultz ever said how they know that the Soviets, whom they described in other contexts as "bold" and "obnoxious," would be so extraordinarily obliging. Considering that Weinberger regularly asserts the Kremlin's willingness to launch a first strike, it requires a gigantic leap of faith to assume that they would humor us by pretending to think that they are in the soup once the MX finds a home.

Failure to go forward would be frightfully costly in other ways, the administration argues.

"The allies would be distressed," Shultz intoned. They would think of the U.S. as a "pitiful, helpless giant" if we fail to throw billions of dollars at a military system that its advocates admit is "a political statement."

The hearings opened in a bipartisan glow. Chairman John G. Tower (R-Tex.) and Jackson were like social workers who have finally found a home for a hard-to-place orphan.

It was Sen. J. James Exon (D-Neb.) who broke the spell. He first unfurled his hawk's credentials--he is for the B1 bomber and chemical warfare--then said of the new scheme:

"It simply makes no sense to me. A wonderful bipartisan consensus does not enhance national security."

Sen. Dan Quayle (R-Ind.) asked why the MX has to have 10 warheads.

"How about five warheads?" he inquired plaintively.

"I suppose that might gain a horselaugh from the Soviet Union," Shultz replied crushingly. Shultz always seems to be passing some unseen macho-loyalty test in his head.

Sen. Carl Levin (D-Mich.), who had noticed that Weinberger, on the whole, was a steadier witness, shrewdly addressed his queries about our comparatively defenseless state to the secretary of state, who, supposedly, is in full charge of our disarmament policies.

Shultz could not answer. Levin taxed him about his contention that we resolved the Cuban missile crisis because of our "nuclear superiority," a claim denied by authorities of the time. Again, Shultz could not answer.

"I assume you know what you are talking about," he said stiffly.

It was an assumption that, at hearings' end, could not be made about either of the witnesses.