Whenever gossip in this town turns to speculation on the possible departure of Secretary of State Al Haig, the candidate most frequently mentioned as his successor is Secretary of Defense Caspar Weinberger. It figures, given Weinberger's background and old-shoe palship with the president. But why bother, I say.

Leave the job vacant; you could save a lot of money in travel expenses and lose nothing. At least half of the time, Cap Weinberger acts and talks as if he thinks he holds both jobs right now.

Not funny? You're right. It is a deadly serious business when the two principal figures in the area of national security are fundamentally at odds on important aspects of strategy and policy. But it is usually manageable--and also traditional. Haig had that last part just right the other day when he conceded there are "clearly differences" between him and Weinberger, but added:

"What's new about that? Each department comes at these problems from their differing perspectives. That's inevitable. It has always been so."

What has not "always been so," however, is the extent to which inherently differing departmental perspectives have been allowed to crystalize into unresolved policy conflicts. What is not "inevitable" is that these conflicts be given public expression in a way that baffles (or needlessly provokes) allies and/or adversaries and confounds the forceful conduct of national security affairs.

In short, what's new about the all-too-clear differences in approach between Haig and Weinberger on the Polish crisis, for one example, or Central America, for another, is the permissiveness of top management. The inescapable implication is that Ronald Reagan believes this public arm-wrestling for influence and preeminence is either (a) of no consequence or (b) unmanageable.

A third possibility, of course, is that Reagan believes that, in a town that dotes on disorder and abhors harmony, a lot of the policy conflict is the work of--you guessed it--the press. That's about half true; it appears in the press. But it gets there courtesy of public as well as private statements by the principals themselves, or the calculated contributions of anonymous subordinates.

And it gets there, in part, out of the natural competitive instincts of bureaucrats with conflicting interests. At the Pentagon, the emphasis is on securing base rights, deploying nuclear weapons, striking up military alliances--and never mind the sensitivities of the host nations, or governments, or the local or regional political repercussions, which are precisely the things the State Department does have to worry about.

The responsibilities and interests of the military and the diplomats, what's more, are inextricably interwined. The neutron bomb is a weapon; its deployment in Europe is a political issue. Trade sanctions are an economic and diplomatic tool; but as they may involve technology of military value, they concern defense planners.

The question is whether these overlaps ought to be sorted out in private or argued out in public. The impulse to the latter course is accentuated in a number of current cases by an exceptionally heavy concentration of hard-nosed anti-communist zealots in key civilian slots in Defense, reflecting Weinberger's own hard line.

The resulting competition with State's more cautious careerists has the effect of inciting brisker competition--and more open conflict.

Personalities add further incitement. Denials to the contrary, Haig and Weinberger are, well, not exactly collegial. Haig's preoccupation with "turf" is legendary. It is heightened by Weinberger's long, almost alter-ego connection with the president. He feels free to hold forth on foreign policy at a length and with a specificity that few old-timers can recall any predecessor having done because he is confident he knows his boss's mind. For his part, Haig cannot be so sure.

>But Haig is supposed to be The Man for foreign policy. And so we see them both racing around the world, sometimes simultaneously. That was the case a week or so ago when Weinberger was trying to strike up tighter ties with Saudi Arabia while Haig was working up a new defense arrangement with Morocco.

Meantime the catalogue of identifiable conflict grows: Weinberger's harder line on Poland; Haig's tougher stance on Central America; the distinctively different emphasis in the approach of the two men to the Middle East--issues on which you would want a settled policy.

Maybe it is, in this instance, unmanageable, even with the installation of a new National Security Council arrangement that at least bears some resemblance to arrangements that have worked before. But you cannot come away from talks with concerned foreigners with the belief that the damage done to orderly and effective American foreign policy is of no consequence.