At the windup of the first six weeks of the joint congressional investigation of the Iran-contra affair, the Democratic chairman of the House contingent, Rep. Lee Hamilton of Indiana, thought one heavy lesson was already clear. It has to do with what happens when frustrations of one sort or another drive a president and/or his administration to scorn the ''normal processes'' and turn to excessive secrecy, dissembling and subcontracting the conduct of official policy to private parties.

What happens, Hamilton said, is that the system breaks down when there is no accountability, no mutual trust and inadequate congressional oversight. The ''full exercise of the process of checks and balances under the Constitution'' is not something that can be remedied by law. It is a question of people, and attitude, he told columnist David Broder.

Now that may strike you as precisely the sort of lofty talk that Jeane Kirkpatrick had in mind when she wrote the other day that critics in Congress and the media are ''seriously distorting the issues and the debate surrounding the Iran-contra matter.'' They are treating it as though it were ''manifestly a legal and constitutional question, rather than a political issue involving Democrats versus Republicans,'' she complained.

That is a wonderfully convenient line of defense for the president's supporters -- always assuming (as the president's supporters do) that he is telling the truth and that next month's high-tension testimony by Oliver North and John Poindexter will not produce a ''smoking gun.'' Dismissing the institutional and constitutional aberrations in the Iran-contra matter makes it much easier to deplore the arms-for-hostages dealing, to concede certain extremism in support of the contra cause, to put it all off to lapses in judgment -- and to dismiss the hearings as exercises in partisan politics.

So who is right about this? Hamilton is -- and not just because he is an especially persuasive and thoughtful fellow. Another persuasive and thoughtful fellow is Hamilton's Republican opposite number, Richard Cheney of Wyoming, ranking minority member of the House committee. Last week, at a breakfast meeting with reporters, Cheney summed up his sense of what we are learning from the hearings. It coincided remarkably with Hamilton's.

Cheney couldn't agree less with Lee Hamilton on Central American policy. ''There aren't two people in the House who are farther apart,'' he says. He thinks that ''obviously there are political overtones to the whole debate'' -- that Democrats are enjoying it in the same way that ''a lot of us on our side of the aisle used to take delight in Jimmy Carter's difficulties.''

But he can't keep up the Kirkpatrick argument ''lock stock and barrel.'' On the contrary, he believes ''the significant stuff is about how the administration decided to embark on this course, why the president made the decisions he did {and} why the decision was made not to notify the Congress.''

There are ''legitimate questions,'' Cheney argues, about the relationship between the president and Congress in the conduct of U.S. foreign policy, and about whether or not the president complied with the legitimate directives from Congress. If that's a problem for the administration, Cheney adds, it's ''one they created for themselves; it wasn't anybody else who decided to send TOW {missiles} to the ayatollah.''

On the matter of secrecy and the ''privatization'' of foreign policy, the two leaders of the House contingent would seem to be of one mind. Asked about his biggest disappointment so far, Cheney said he had this impression: ''Important decisions were made without understanding they were important -- the decision, for example, to opt for the private support system for the contras rather than to go public.''

He starts with the proposition that the ''tough part of foreign policy is not figuring out what to do overseas; it's building domestic support at home to sustain the effort.'' The ''major foul-up'' was the way in which the administration embarked upon ''an extremely controversial course of action'' without thinking through its long-term implications ''for building public support'' for its policies elsewhere in the world.

The policies Cheney would seek public support for are not necessarily the same ones Hamilton would have in mind. No matter; it's the principle of the thing. For now, it's enough that Democratic and Republican leaders on the House committee are already able to rise above partisanship, see beyond the grubby details and find the same useful lessons in the Iran-contra affair.