FIDEL CASTRO has done few things nastier than slipping several thousand criminals and other misfits into the group of 125,000 people that he allowed to set sail for Florida from the port of Mariel in 1980. Humanitarian feeling and public pressure led Jimmy Carter to welcome them all with "open arms," without screening them one by one as governments normally do. Though most of the Mariel group fitted in fine, the misfits have inflicted a difficult burden. The Cubans have rebuffed the United States' quiet efforts to return them and so the State Department had some reason to go public and to demand that Cuba now take them back. Otherwise, State said, the United States will deny immigrant visas to all but the immediate relatives of U.S. citizens.

No doubt the United States can cause Cuba a certain embarrassment by reminding everyone of the nastiness of 1980. The Cubans may also be concerned that the United States is closing off one part of its traditional emigration safety valve. We wonder, however, whether the new tactic is likely to be effective or wise.

In the immigration categories that would now be formally closed, visas have not been given anyway since the Mariel days of 1980; this was done after Cuban police hounded some 400 desperate visa- seekers into the American mission in Havana. Nor is it apparent that the United States has any new leverage that would make Cuba remove the cynical conditions it has attached to the return of the undesirables.

One condition is that the misfits return voluntarily. But presumably few would voluntarily go, in which case the United States would be in the position of qualifying its reluctance, traditional since the Korean War, to return people against their will to communist countries. The other Cuban condition is that Havana be able to veto returnees on an individual basis.

But there is a more immediate problem arising from the painful fact that to use immigration policy as a stick means that certain innocent individuals are going to have to pay a heavy personal price for political considerations over which they have no control. Specifically, there are in Cuba 1,500 or more former prisoners--political prisoners, most Americans would call them--who are ready to come to the United States but who cannot get the necessary American papers. Some of this number appear to fall in the non-immediate-family categories that have been excluded by undeclared American practice since 1980 and that would be excluded by declared American policy now.

We do not think it is worth beating Fidel Castro over the head with the Mariel undesirables if it means that an opportunity is lost to allow far worthier Cubans to come to the country of their choice.