''I just find it hard to accept on moral and political grounds that we should give aid to a country that is killing its people.'' These words from a US congressman visiting El Salvador represent a sentiment that President Reagan by law has to address very soon. He is expected to do so by arguing that the Salvadorean regime's forces have been committing fewer political killings than in the past.
Mr. Reagan will make this point as part of his imminent certification that the Salvadorean leaders are satisfying Congress's political and economic requirements for continued US military aid. The task of Congress is to scrutinize his findings in the light of its legislative goal: to ensure that US aid serves the cause of reform and not its opposite.
Would there be a useful shock in cutting off aid until dramatic reforms are achieved? Or is there more hope for the people of El Salvador in the present approach of encouraging at least marginal gains through continuing aid even on the basis of controversial Washington certification?
The effectiveness of sharp congressional outcry was shown when aid cutoffs were threatened over suspension of land reform in El Salvador. Now Mr. Reagan is expected to certify that land reform is going ahead with the help of army enforcement.
But there are arguments for continuing conditional aid through at least some ups and downs of reform: It may be all that has kept El Salvador's totalitarians from taking over completely from less extreme elements. And US aid has coincided with some promising steps that Mr. Reagan can point to. These include the establishment of a Salvadorean human rights commission and plans for a presidential election next year. US aid might provide some leverage to keep that election a legitimate expression of the people's views.
Still, the Salvadorean authorities have given little reason for confidence - witness their delaying tactics in handling the murders of US churchwomen and others.As for the deaths by ''political violence,'' they are down by all accounts. But they are calculated to exceed 2,000 in 1982 even according to conservative US Embassy figures.
Will Congress find the net situation showing enough progress for continued US aid? Probably yes, if the past is a guide.But there is some thrust for reconsidering the certification law, which expires in the fall. Perhaps new conditions could be spelled out in it, such as a requirement for progress toward unconditional negotiation with the rebel forces.
Unless manifest progress is made, the question of why the US should aid a government that kills its own people will become more urgent.