By leading the U.S. Congress to its vote to cut off aid to the democratic resistance in Nicaragua, a small band of liberals has assumed a grave moral obligation: the responsibility to persuade Nicaragua's rulers to demonstrate that only the Reagan administration's policies have kept them from fulfilling the democratic promise of the Nicaraguan revolution. Now that those policies clearly are no longer in effect, there can be no more excuses.

A foreign observer could see that last week's vote was the result of many factors: partisan bitterness, procedural confusion and a lack of public understanding here about the history and significance of the Nicaraguan civil war. But at the core of congressional opposition to the aid was a moral and intellectual assumption that evidently still guides American liberalism: communists would become democrats if only they were more generously treated.

This still remains the conviction of the Vietnam generation: leaders such as Reps. Michael Barnes and Jim Leach, and Sens. John Kerry and Tom Harkin. They are confident that they possess a unique sophistication and moral authority that, if only given its way, can win over the Sandinista comandantes. It is a conviction I know well -- too well. It has cost me five years of bitter disappointment and two attempts on my life to learn how profoundly untrue it is.

I myself am one of the Vietnam generation, and I too believed that communists could be persuaded to respect pluralism, human rights and peace. Many of us joined the Sandinistas in a National Unity Coalition against Somoza, but when he was overthrown in 1979, the Sandinistas threw away all our carefully negotiated agreements for democracy and peaceful reconstruction. I nevertheless served with them on the governing junta, but was obliged to resign when it became clear that I was a figurehead with no authority. I then spent two years inside Nicaragua as a revolutionary dissident, but learned that the Sandinistas do not tolerate political dissent. Tom Harkin, then a congressman, helped me get out of Nicaragua into exile. There I helped organize appeals to the Sandinistas from European socialists and Latin American democrats. It has come to nothing.

The young liberals of the United States believe they can now accomplish what so many others could not: persuade the Sandinistas to join the community of responsible, civilized governments.

I hope that their consciences -- and the pressures of your own political system -- will now oblige Messrs. Harkin, Kerry, Barnes and Leach to go to Managua in order, as Barnes put it, to "call the Sandinistas' bluff" and to execute the moral obligation their legislative victory has created. In exchange for cutting off aid to our cause, they must receive specific and significant concessions from the Sandinistas -- not just fine print and eventualities. An honest concern for peace and human rights cannot be satisfied unless:

* The Sandinistas agree to the Nicaraguan bishops' call for negotiations with both the civic and armed opposition.

* The Sandinista People's Army -- now the army of one political faction -- becomes a genuinely Nicaraguan army.

* The Sandinistas accept international observers who can guarantee that pluralism and a cease- fire are respected.

* An ironclad and prompt timetable for implementing those agreements is established.

If the young liberals in Congress can obtain these concessions from the Sandinistas, we will all be obliged to honor their insight and ability. But if they cannot, they must have the integrity to join me in the painful admission that they were wrong, and that the struggle for democracy in the Third World cannot be fought with the communists, it must be fought against them.