The underlying assumption of most senators who were about to confirm him as secretary of state was that George P. Shultz had to be kidding when he said he supported unequivocally the cap-over-the-windmill economic foreign policy of the man who appointed him.

It was flattering to him but most derogatory to Ronald Reagan, and the secretary-designate was obviously embarrassed as his judges kept telling him they knew he was smarter than he was letting on.

The stately Shultz was at considerable pains to conceal his formidable intelligence, and the sophistication and experience that are going to make him a heavy in the Reagan Cabinet. He has, he insists, no views other than those of "the boss."

Things reached a head on the second morning of his hearings yesterday. Foreign Relations Committee Chairman Charles H. Percy (R), whose home state of Illinois is suffering considerable misery over the president's reignited resistance to the Soviet pipeline, pleadingly assured Shultz that he is counting on him to "fight for what you think is right," to reverse a policy that has cracked the NATO alliance and cannot stop construction of the pipeline.

It was an especially awkward moment for Alexander M. Haig Jr.'s successor, and he respoke, unconvincingly, his whole-hearted agreement with the president about "an overriding concern for the plight of the Polish people" that Reagan's sanctions are somehow supposed to alleviate.

He could not take issue with Percy's premise. Shultz has written down his strictures against "light-switch diplomacy," and key lines were quoted back to him several times. He is an economist, a free-trade advocate who has belabored the folly of losing, not just money, but the nation's reputation as a reliable trading partner by off-again, on-again trade practices.

"I am just trying to say," he said lamely, "that in these matters not everything important is in the economic sphere."

It was hardly an utterance befitting the erstwhile president of one of the megadollar multinational corporations, the Bechtel Group.

But Shultz' role before the committee seemed to be to appear less intelligent than he plainly is and to possess less substance than he plainly has. He didn't mind appearing to have been born yesterday, a difficult pose for a veteran of the Nixon administration.

He fairly bragged about his lack of current information. "I am not in the decision loop," he frequently and gladly told the senators. The cramped style was adopted in the interests of displaying his gift for team play, for blandness and conformity.

He was under the impression that his biggest job, besides persuading senators that he had put Bechtel behind him, was to show the committee that he is not Haig. He didn't need to try so hard.

The differences are dramatic. For openers, Shultz speaks English. Haig had many enemies, real and imagined, but against none did he wage more cruel war than his fight with the mother tongue. It is impossible to imagine Shultz speaking, as Haig once did, of "the vortex of cruciality." Nor would he be guilty of the metaphorical equivalent of Haig's cluster-bomb attack as in, "I don't want to saddle myself with a statistical fence."

Shultz' command of the language, his calm, measured paragraphs, offered as compelling a contrast as the image-makers in the White House could have asked. In case anyone missed it, Shultz subtly underlined it. He feigned reluctance to see a transcript of the hearings because "I make such a hash of the language."

He knows that not to be the case. He is a dignified, imposing man, a cross between a British general and a German banker. Sitting down, he looks taller than he is.

He is wonderfully composed except when someone casts aspersions on his old company, as he thought Alan Cranston (D-Calif.) had done when inquiring into Bechtel's peddling of nuclear reprocessing equipment to Brazil at a time when the United States was beseeching West Germany not to make a similar sale. Shultz took heated exception to what he called Cranston's smear.

It was his one ruffled moment. Mostly he was fending off "say-it-ain't-so" appeals. Richard G. Lugar (R-Ind.), in a melancholy soliloquy about the lunacy of Reagan's refusal to sign long-term grain sales agreements with the Soviets, invited Shultz to agree with him that "it is a disaster for American agriculture."

Shultz couldn't. All he could say was, "I think the situation in Poland is important."

It is lock step time at the White House, at least for him. None of the frustrated senators noticed that for another member of the Cabinet, it is not. Agriculture Secretary John R. Block is conducting a noisy public campaign to have the grain sales decision reversed.

But Shultz swears Reagan's foreign policy is "consistent." For what it is worth, few believe that he believes it.