Ollie North has at last committed an unpardonable sin: He's let down Sen. Orrin G. Hatch. The Utah Republican up to now has stood fast with North and his Merry Band of Constitution Manglers. But the other day there came testimony so awful, so terrible, so damning, that Hatch had no choice but to express his "feeling of being let down." North had tried to cover up the fact that he had accepted a free security system for his home.

To Hatch, this was too much. Like some other Republicans on the committee investigating the Iran-contra affair, he had been unmoved by what he'd learned so far: The president lied about not trading arms to Iran in exchange for American hostages in Lebanon and about the role of a "third country" (Israel); he signed a law (the Boland Amendment) and then circumvented it; he misled Congress about his intention for Nicaragua and has showed himself serene about revelations that his subordinates franchised American foreign policy and were reaping the benefits of their entrepreneurial skills.

Hatch is tone deaf to sounds that prompt others to yell bloody murder. Elliott Abrams, an assistant secretary of state and the so-called "point man" for the anti-Sandinista effort, misled Congress about the secret funding of the contras in general and about a $10 million gift from the sultan of Brunei in particular. The former national security adviser, Robert McFarlane, had been a virtual smoke machine of lies and cutesy statements. To all this Hatch and others registered bored indifference.

Hatch's sudden letdown comes a bit late in the game. But it is in consonance with the ethics du jour of certain conservatives who, it turns out, are hardly conservative at all when it comes to law and the Constitution. For their cause -- for anticommunism in all its myriad forms and fantasies -- they countenance with equanimity example after example of the law's being flouted or ignored.

The other day, for instance, conservatives convened to memorialize William J. Casey, the former director of the CIA. From the available evidence, it was Casey who master-minded the secret funding of the contras. He was Daddy Warbucks to Little Orphan Ollie. The Casey crowd hardly paused to consider the damage its hero has done to the very agency he headed or to the Constitution to which he swore fealty. If he thought he was right -- and they agreed -- neither law nor honesty could stay his course. At that event, the lovely lemming, Fawn Hall, was cheered for blindly following North to the shredder.

Law and honesty matter. They are the bricks by which democracy is made. The Constitution does not mention sincerity of purpose as an excuse for breaking the law. It does not matter that Reagan is sincere in his anticommunism, that he thinks Marxist Nicaragua is a real, if congressionally unrecognized, menace. He still had an obligation, both ethical and constitutional, to tell Congress and the people what he was up to and to publicly, not secretly, challenge laws with which he disagreed. No matter what he might think, no matter how lofty his purpose, he is not above the law.

As for North, what's the big deal about covering up the acceptance of a gift? He had already, in fact or spirit, broken so many laws, what did a mere security system matter? Doesn't Hatch understand? The security system, like snow tires or a burst of shredding madness, was part of the cause, installed in the name of the cause, turned on and off to advance the cause. The law says a federal official is forbidden from accepting gifts from private citizens. But North had already shredded the law, not to mention a record of his activities. In this, he was cheered on by some committee members investigating him. The end -- an anticommunist victory -- would be well worth the means.

Maybe North's security system serves a symbolic purpose. By having it erected, North has forced some politicians to understand that fences are what this scandal is all about. Jewish sages advocated building "a fence" around religious law -- to be meticulous in observance lest, through erosion, the law itself be broken. Some time ago, the Reagan administration smashed through that fence and stepped on the law itself. Now, maybe, North's more prosaic fence illustrates the damage done. It increased his security but at the cost of our own.