On the fourth day of his testimony, Oliver L. North described what might be called The Casey Concept, the brainchild of North's mentor, the late CIA director William J. Casey. The idea was to use earnings from commercial undertakings like arms sales to Iran to create a "stand-alone, self-financing" secret agency capable of conducting worldwide covert operations without reporting them to anyone.

Marine Lt. Col. North's disclosure, drawn forth under sharp questioning by Senate chief counsel Arthur L. Liman, was revealing and instructive -- revealing of the Reagan administration's secret ambitions and instructive because of the emotional reaction it provoked from members of the Iran-contra committees.

A heated debate flared briefly in the committee room, witnessed by millions watching on television, that suggested something more than fatigue and frayed nerves at work on Capitol Hill. It was a sign that the underlying issues of the Iran-contra affair -- secrecy, covert operations, governmental accountability, ideology -- are certain to figure in political debates long after the hearings are history.

It was also evidence of how powerfully North has affected the hearings and of the strongly polarizing political effect of his testimony. Not only committee members are taking sides over Oliver North. To judge from the public reaction thus far, so is much of the country -- though an ultimate verdict on this charismatic Marine probably remains far in the future.

Yesterday's heated exchange ostensibly was prompted by objections from some Republican House committee members over Liman's relentless inquiry into what North knew about the secret network planned by Casey. In fact, the debate was about the nature and wisdom of covert operations in general.

The dispute began shortly before the luncheon recess when Rep. Bill McCollum (R-Fla.) interrupted Liman's questioning of North and strongly reprimanded him.

"I think Mr. Liman is out of line asking questions that prejudge opinion of this committee," McCollum said. "He is phrasing his questions to make an argument to slant it as though the entire committee thinks that this is a horrible thing. He doesn't speak for everybody . . . ." Rep. James A. Courter (R-N.J.) echoed McCollum's criticisms.

But after lunch another Republican, Sen. William S. Cohen (Maine), took a different tack: "I reject the notion that somehow because the members don't like either Mr. Liman's tone or style that he should be forced to cut short his questioning." Cohen described the plan North disclosed to create an off-the-books intelligence capability "with or without presidential findings, with or without notice to Congress" as "perhaps the most serious revelation to have taken place during the course of these proceedings."

Added Cohen, "If members of Congress are not disturbed about the revelation, then I think the American people should be. And if it takes more time to discuss this in depth and other related issues, I am perfectly happy to yield whatever time I have allocated to me so that Mr. Liman might continue. But I strongly object to the notion raised by House members of trying to impose a gag rule upon Mr. Liman."

The flash of emotion exposes potential problems for Republicans in dealing with North and with the fervent defense of covert operations he espoused all week.

For much of the week, North's star appeared to be rising. He got high marks for his compelling television personality and the picture he presented of a single good soldier battling on alone after being abandoned by his superiors.

His performance reassured original North supporters among the Republicans on Iran-contra committees after doubts about his motivations had arisen during earlier testimony. But for Republicans, embracing North too warmly now poses a problem. If he helped himself this week, he did so at the expense of his old bosses. His testimony has been devastating to the high command of the Reagan administration. Each day brought new disclosures about high-level involvement in assorted lies and coverups, and have made it difficult to disassociate North from officials still running the government. His most effective defense, it seemed, was his persuasive insistence that he always had his superiors' authorization.

Yesterday's disclosure of The Casey Concept contained another irony. North had testified that CIA director Casey took special delight in the idea of diverting the ayatollah's money for the contras, calling it, in North's recollection, "the ultimate covert operation."

The committees and public learned yesterday that North's National Security Council boss, Rear Adm. John M. Poindexter, became concerned that the Marine lieutenant colonel was becoming "too public," and ordered him to talk to no one "except me" about North's covert operations. Poindexter specifically ordered North not to talk to Casey about these matters, according to yesterday's testimony. This led Liman to observe wryly:

"So this business of covert operations reached a point where not only Congress was regarded as too indiscreet to be told, but that even the director of central intelligence made that list."