Senator Percy's warning to El Salvador's rulers ought to be heeded by his fellow Republican in the White House as well. For President Reagan's hope of continuing heavy military and economic aid to those rulers depends on their meeting Congress's conditions for it. And Mr. Percy responsibly warned that ''not one cent'' will go to them if they renege on such a key condition as land reform. He was responding to last week's decision by El Salvador's new constituent assembly, under the leadership of extreme right-winger Robert d'Aubuisson, to suspend if not abolish an important part of the land reform program.
If the suspension is carried through it would be extremely awkward for Mr. Reagan. This summer he is required to certify once more that the Salvadoran regime is making sufficient progress on human rights to meet Congress's criteria. His last certification was roundly challenged in the light of widespread abuses, but Congress went along. It will hardly do so again if even land reform, on which the administration has placed so much weight, should be held down.
To be sure, the three-phase land reform program, launched in 1980, has been found wanting in equity, adequacy, and effectiveness. One phase has never been implemented, the conversion to cooperatives of medium-size farms that produce half the nation's sugar, more than half its cotton, and almost 90 percent of its coffee. But peasants were offered new rights under the ''land-to-the-tiller'' law by which sharecroppers or tenant farmers are allowed to buy up to 17 acres of the land they have been working. It is this phase that Mr. d'Aubuisson and the assembly would suspend.
Whatever the shortcomings of Salvadoran land reform, Senator Percy and others correctly see such a setback to it as a bad signal. If US aid is ''indispensable ,'' as El Salvador's new President said last week, Mr. Reagan has an obvious opportunity. He can expect the rulers to meet the full standards for certification, including land reform, if they expect him to give them a clean bill of health on Capitol Hill.