Congressional lawyers investigating the Iran-contra whatever may still be able to spin a web of conspiracy charges and catch a witness, but that looks less and less likely all the time -- even to those working hardest at it. No one should feel too badly about that, since Congress is not a law-enforcement agency. It is a lawmaking agency, and therein lies a moral.

It is not too late for these hearings to produce something of redeeming social value -- if the talented team of Democrats on the committee would turn its attention to the policy problems involved.

It is the job of a responsible opposition not only to oppose but also to offer realistic alternative policies. Therefore, it is surely time for the distinguished Democrats on the committee (and they are distinguished Democrats) to tell us what they believe the U.S. government should do about Middle Eastern terrorism and about communist Nicaragua.

What do the Democrats on the committee recommend we do about Terry Anderson, Terry Waite, Charles Glass and others being held captive, and about the taking of hostages, which almost everyone now agrees is an Iranian form of war very likely to continue?

Answers that sound good but won't work are not really answers. ''Take it to the United Nations'' is not an answer. Everyone knows, or should know, it won't work. ''Take it to the allies'' won't work either. Each of our European allies works alone to free its own citizens. As the recent collapse of European Community sanctions against Syria demonstrates, the allies have a great deal of difficulty coordinating action against terrorism.

Perhaps someone on the committee thinks we should bomb Tehran to demonstrate the cost of victimizing Americans. Or send commandos on a rescue operation. Or take captive the Hezbollah families within our reach. Or forget about the whole thing.

Maybe some Democrats on the committee think we should respond to Iranian terrorism by helping Iraq win the war. Maybe some think we should try negotiating a deal with the Iranian government. Perhaps they think we should try to punish Iran by wooing away its only regional ally -- Hafez Assad. That seems to be what is currently under way.

Although Britain appealed to its fellow EC members to continue sanctions against Syria until there was clear evidence Assad had abandoned terrorism, Denmark has restored high-level relations, and the Federal Republic of Germany has restored economic assistance.

Presumably U.S. Ambassador Vernon Walters took Assad neither a cake nor a Bible, but he did travel to Damascus to have two days of ''good'' talks with Assad shortly before Syria's foreign minister visited Tehran to offer full assurances of solidarity. Now it is said a U.S. ambassador will be returned to Syria this fall.

My own guess is that the current American-German-Danish strategy vis-a-vis Syria has about as much chance of success as the arms sale to Tehran. But never mind what I think. What do the Democrats on the committee think?

And what do they think we should do about Nicaragua's repressive, expansionist government? It is another problem that will not go away. Ronald Reagan believes there are moral and strategic reasons for the United States to support Nicaraguans who oppose their Marxist rulers. Most of the time a majority of Congress has agreed with him. But for a brief period a majority imposed prohibitions on U.S. intelligence agencies offering assistance to the contras.

Sensible people -- including the opponents of the contras -- now understand that military forces cannot be disbanded and re-created at the whim of legislators. When and if the Nicaraguan resistance is dismantled, there will be no more resistance to the Sandinista military machine. The comandantes will then be free at last to use their full force, unopposed, against the people of Nicaragua. Congress itself has changed its mind about the wisdom of a policy that permits such an outcome.

It would be very helpful if the distinguished Democrats on the committee undertook a realistic discussion of what the U.S. government should do about the government of Nicaragua. Such a realistic discussion would, of course, take account of Soviet electronic installations in Nicaragua that intercept communications through the Panama Canal, of deep-water port facilities under construction and of a runway long enough and strong enough to accommodate fully armed Soviet bombers. It would also take account of the repeated and unsuccessful efforts to negotiate with Nicaragua an end to the importation of heavy weapons and foreign advisers, and to reach an internal reconciliation.

Many of the Democratic congressmen who as recently as 1980 assured us the Sandinistas were democratic reformers will still tell us today that we have nothing to fear -- from a Marxist Nicaragua or a Marxist Central America or from Soviet bombers.

But most other Democrats -- and most Republicans -- in Congress will doubt that and understand the danger of taking such risks.

It would really be useful if, after hearing all this testimony, the Democrats on the committee would tell the American people just what they think the U.S. government should do about these very difficult situations.