THE SUCCESSFUL U.S. military action against an Iranian ship caught laying mines in the Gulf is forcing fresh consideration of whether the War Powers Act should be invoked. That's the Vietnam-era law enacted by Congress (over a Nixon veto) to legislate itself into a role in committing American combat forces and to prevent presidents from entering any more undeclared wars. No president has since accepted its application anywhere, and President Reagan, claiming infringement upon presidential prerogative and policy discretion, stoutly resists its application to the situation in the Gulf. But now that the American military has struck Iranian forces and Tehran is threatening retaliation -- events seeming to meet the law's trigger of ''imminent'' hostilities -- there are new demands in Congress to compel formal invocation of the act.

These demands, however, are not the half of it. Only last Friday the Democratic-controlled Senate voted 50 to 41 against invoking the law. Legislators have varying views, not necessarily corresponding to party, of whether the Gulf prospect is adding up to the sort of war that the act anticipated. There is fear that invoking the law now, even in circumstances suggesting Congress would approve of presidential policy, would send mischievous midstream signals of confusion and inconstancy. Congress is demonstrably reluctant to grasp the responsibility of shared decision-making that the War Powers Act thrust upon it and to accept the political risk of either halting or endorsing Mr. Reagan's policy.

The purpose of the act was to ensure consensus -- and the broad consultation, close inspection and considered pace that produce it -- in crucial national decisions of war and peace. The evident fact, however, is that application of the act here could create as much confusion and dispute as consensus -- in a context where there already exists a considerable and informed consensus around the president's policy. Some substitute formulas of consultation are brewing. There is no magic in them, but there is no gratuitous division in them either. That would leave for another day consideration of what to do about a War Powers Act that now stirs fierce resistance at one end of Pennsylvania Avenue and only tepid support at the other.