NOT ONCE but twice, Sen. Howard Metzenbaum suggested the other day that Libya's Muammar Qaddafi ought to be assassinated. Many people would applaud the remark or at least condone it on grounds that he was only voicing the general revulsion. But he was doing more. He was validating Col. Qaddafi's own idea that assassination is a reasonable way to work off a political grievance.

A great many people around the world are not going to pay close attention to the Reagan administration's prompt reiteration that assassination is outside American law and policy alike. They are going to remember that a U.S. senator put an American imprimatur on the notion of resolving political conflict by killing the head of state. Among the possible consequences could be a greater determination by the allies to put distance between themselves and the impatient Americans.

Economic sanctions, to be sure, have their limitations. The allies have made clear they have no intention and no political leeway to go along. Prudently, the United States is trying to avoid making its sanction call into a crusade; that would risk turning a confrontation with Libya into a conflict with the allies.

Instead, the Reagan administration is concentrating for now on ensuring that the allies do not fill the gap left by its latest measures of economic withdrawal. There can be no illusions that Col. Qaddafi, who has shown he will not be shamed out of terrorism, will be squeezed out of it. Still, the experienced American oil and construction hands who may leave Libya cannot be replaced overnight. In a time of an oil glut the loss of the American market is not insubstantial.

The American effort should be to stay involved in building support for boycott measures so long as Libya's policies do not change. Steadiness, not a brief burst of concern, is the best basis on which to bring along the allies.

Zbigniew Brzezinski suggests on the opposite page that the Europeans might become even readier to join sanctions if they felt that, for the United States, the alternative to collective economic action was to embark on some sort of unilateral military course or if they saw some genuine Soviet strategic threat developing in Libya. No one can be eager to test these propositions, least of all the allies. Their very dread should make them consider the relative advantages in notching up the economic pressure on the Libyans. There are a dozen reasons why the Europeans want to hang back. There are some very good reasons why they shouldn't.