IT DISTURBS MANY people to think of a loose cannon like Rep. George V. Hansen (r-Idaho) rolling around Tehran conveying to Iranians not well versed in the American political scene that, as the State Department put it, "there may be more than one voice speaking or negotiating for America." Mr. Hansen was in Iran on a self-launched "personal mercy mission." His proposal for a congressional inquiry into the former shah's alleged iniquities was unauthorized and could concievably have led some Iranians to believe they could make an end run around the administration's demand for the immediate release of all hostages. For that reason, it was good that his House peers repudiated him quickly.

The more one contemplates the performance of Mr. Hansen, however, the harder it is to single him out for criticism. Leave aside private citizens: he is far from the first legislator to assign himself a diplomatic mission in order to compensate for the perceived inadequacies of the administration in power. Assorted sojourns in Hanoi, Moscow, Havana and Beirut (to see the PLO) come to mind.

It is hinted that Mr. Hansen's ego or his political ambition had something to do with his trip -- a contention that some of you may find hard to believe about a politician. The fact remains that not so long ago a congressman proceeding to the capital of a hostile state without official instructions or approval, even (or especially) during a crisis, could expect to be hailed as something of a hero in broad sectors of American society. If the congressional traveler conveyed "that there may be more than one voice speaking or negotiating for America," then that was considered to be precisely the point of the excercise. Of just such stuff have congressional-executive relations in the foreign policy field been made for a dozen or more years.

All this is not to say Mr. Hansen was not a jackass to take himself off to Tehran. It is to say that he is in a large company of congressional Walter Mittys pretending to be secretary of state. Presidents find this state of affairs maddening, notwithstanding the view they had when they or their party were out of power. Congress finds it statemanlike to rally around the president until it becomes necessary to assert a national interest that the president, poor fellow,is leting trail.

In fact, the Hansen affair seems egregius enough to enough people to help the president's -- any president's --side of the foreign-policy tug of war. But it hardly seems enough to end it. In recent years politics has not so much stopped as started, or at least intensified, at the water's edge, and that will not soon or easily change.