Sen. Edward Zorinski (D-Neb.) has a loose hold on a weak idea. To build more coherence into the conduct of American foreign policy, he would make the president's national security adviser subject to confirmation by the Senate and cross-examination by Congress.

Hence the Zorinski amendment, which cleared the Senate Foreign Relations Committee last year and sailed through the Senate on the wings of a State Department spending authorization bill. It was knocked out in conference with the House, under pressure of a threatened presidential veto. Now it's back and so is the argument.

To understand it, take an example that comes readily to hand. Suppose that the incumbent adviser, Zbigniew Brzezinski, were to be hauled before Congress to account for some suspected departure on his part from apparent official policy. Would he not be forced either to conform or to confirm a schism in the policy-making high command.?

Under the circumstances, he would be powerfully inclined to conform -- or to waffle. If the effect were thus to reinforce the impression of a United States speaking with one voice on foreign policy, that would facilitate domestic as well as international understanding of American policy.

But Sen. Zorinski must know better than anybody (except any other senator) that this would leave a large part of the problem unresolved. For precisely the reasons that impelled him to offer his proposal -- a widely shared congressional yearning for a stronger hand in foreign policy-making -- the U.S. government, collectively, speaks only occasionally with anything like one voice.

Presidents Johnson and Nixon, for example, had one view on Vietnam; over time, Congress developed another. We all know which voice Hanoi was listening to.

It would still be helpful, of course, if at least the executive branch regularly offered just one version of the policies the president presumably prefers. But the problem with the Zorinski solution is that it wouldn't even accomplish that. There will always be the familiar babble of calculated "background" leaks to ventilate dissent.

Still less would Zorinski's proposal achieve the secondary purpose of giving Congress not only its customary chance to question the official line laid down by the secretary of state in congressional hearings, but also to dig out for public display any contrary counsel that the president might be receiving from his special adviser for national security affairs.

That's not to say that Congress couldn't establish the security adviser's job by law. (It now exists by presidential decree.) But having done so, Congress would have to wonder whethers it was creating anything of consequence. The president could leave the post vacant, or fill it with a yes-man.

To his credit, Zorinski concedes that the president is free to go about making foreign policy as he wishes. Further, he agrees that he may be generalizing from a particular state of affairs that gives him offense. He says that he acquired "from confidential sources" two documents -- one having to do with banning nuclear weapons in Latin America and one having to do with U.S.-Mexican relations -- that convinced him that Brzezinski has been a maker of policy in his own right, not just a coordinator of the making of policy. That being the case, Zorinski, a member of the Senate Foreign Relations Committee and something less than a fan of Brzezinski, wanted him on the witness stand. The White House invoked executive privilege for confidential advisers, something it couldn't do if the position had been filled with the Senate's advice and consent.

In the course of recent hearings on Zorinski's amendment, the administration reitered its opposition, with near-unanimous support from witnesses with firsthand experience in the foreign policy-making business. Yet the same witnesses almost all agreed that there's a problem -- that the position is wide open to abuse.

If Zorinski's answer isn't the right one, what is?

Former holders of the White House job more or less agree with the view of McGeorge Bundy (under Kennedy and Johnson) that the senior man on foreign policy at the White House should not become a "major public exponent of particular opinions. . . It is an inside, not an outside job."

And if the top foreign policy man at the White House disagrees with the secretary of state? Walt Rostow, Lyndon Johnson's White House man after Bundy, thinks the solution has to lie in the relationship between the two. He and former secretary of state Dean Rusk had a prior understanding, faithfully fullfilled, that if Rostow offered a view to the president in conflict with State Department thinking, Rusk would be alerted to it in advance.

The consensus, in other words, is that Zorinski is wrong in advancing a statutory remedy and dead right in his deepdown realization that, in the last analysis, how well the system works is something Congress can't legislate. What Congress can do -- and what I suspect Zorinski and his supporters are really trying to do -- is to put a little more pressure on the president and his principal foreign policy advisers to clean up their act.