Sen. Patrick Leahy, former vice chairman of the Senate intelligence committee, had no defense when one copy of the panel's preliminary report on the Iran-contra affair was leaked to NBC last Jan. 8 with the word ''declassified'' scribbled in the corner of the title page.

A committee staffer noted that when copies of the report were recalled after the panel voted not to release it, Leahy's was the one with that word on it. The word showed clearly when the leaked copy was flashed on TV screens. If he had claimed innocence, the handwritten note would have cemented his guilt, ensuring his expulsion from the committee. Instead, he resigned.

That not one word was said to discourage Leahy from resigning was remarkable enough in Capitol Hill's most notorious leak mill. What followed was more remarkable: a tough decision by the new chairman, Democratic Sen. David Boren, and the new vice chairman, Republican Sen. William Cohen, to close the mill. Committee members are on bipartisan notice that leaks will not be tolerated.

The best evidence of this was the decision of Boren and Cohen, six months after Leahy's resignation, to make public the formal Senate ethics committee finding last week that Leahy had been responsible for leaking the January report.

They wanted the public to know about the ethics committee's finding because they wanted credibility for their attempted transformation of the intelligence panel from leak mill to protector of the nation's secrets. Taking the issue before the full intelligence committee, they asked for a vote. Unanimously, it ordered that the ethics finding against Leahy be released, guaranteeing maximum publicity and humiliation.

Shrewd, hard-working Pat Leahy once demanded a personal CIA briefing at 5 a.m. before going on a morning network news show. While the late CIA director William J. Casey repeatedly pleaded with him for discretion, his propensity for playing big-shot intelligence insider to the news-hungry public was not all that different from other senators'. What gave him such wide latitude was complicity of the chairman at that time, Republican Sen. David Durenberger.

Like Leahy, Durenberger was not sensitive to how leaks were sinking sensitive projects. The leak of a joint U.S.-Egyptian operation against Libya probably cost the lives of several Egyptian intelligence agents and aborted the operation.

Durenberger is currently the target of a highly unusual ethics committee probe. His office declined to confirm or deny to us a report by Capitol Hill sources that the senator has retained an attorney to represent him in possible future actions against him by the committee.

Durenberger's apparent transgression was a charge he made before an American Jewish audience last March 15, two months after his intelligence committee chairmanship ended. He accused Casey of having ''changed the rules of the game'' by authorizing a CIA spying operation against Israel following its 1982 invasion of Lebanon.

His apparent intent was to diminish American anger at Israel for recruiting Jonathan Jay Pollard for espionage. The allegation against Casey, if accepted, made it seem that Israel had recruited Pollard in self-defense.

In the political explosion that followed, Durenberger never denied he had made the charge. The incident was quietly referred by Boren and Cohen to the ethics committee.

While it is far too early to forecast whether the Boren-Cohen effort will work, the sincerity of the effort cannot be doubted. Administration or committee-drafted ''documents'' no longer can be checked out of the committee's secure Room 219 of the Hart Senate Office Building. Documents must be read there under the eyes of two security agents. Nor can senators any longer take away their notes of top-secret briefings. The notes must be deposited in a safe whose combination has a security classification as high as a CIA secret.

Skeptics have plenty of doubt whether these new rules will actually stamp out dangerous leaks, but Boren and Cohen have started out on a straight and narrow track.