IT HAS BEEN a long uphill climb for the United States, over two administrations, to draw attention to the Soviet Union's responsibility for chemical warfare violations -- in Southeast Asia and in Afghanistan. The interesting question is why the general response has been so slow.

One reason has been the difficulty of assembling evidence to meet the standards required to sustain such charges. The scenes were remote, the victims simple people for whom "yellow rain" was but one of many travails. In the world at large, many people found it hard to believe the Soviet Union would sponsor and practice a barbarous form of warfare it had vowed in two treaties to forgo. Did people dismiss the threat on such disreputable grounds as that it mainly concerned more primitive folk or raised too many disturbing questions about the Soviet Union's worthiness as an American negotiating partner? Governments shied away from lending support to a charge that might be thought to arise from an American Cold War campaign. Nor did this administration help its case, always a difficult one to prove, by the manner in which it initially presented it.

It seems to us now, however, that the administration has proven out the Soviet pattern by a standard that reasonable people would accept. There are the blood and urine samples, there is the anecdotal evidence of refugees and survivors and -- the latest -- the receipt of toxin-contaminated Soviet gas masks from Afghanistan. As recently as six weeks ago, the State Department reports, Soviet forces were using lethal chemical weapons there. An international public that could weep for Lebanon surely can mourn the evident thousands of victims of Soviet chemicals in Afghanistan, Laos and Cambodia.

Soviet conduct has already had its impact in reinforcing American suspicions of the Kremlin. The impact elsewhere is less evident, notwithstanding the administration's efforts to make the issue more universally acceptable by avoiding presentation of it in stark East-West terms. Still, last Friday the United Nations did agree by a large margin to convene the parties to one of the international treaties Moscow has evidently broken, the biological weapons convention of 1972. Just this week, the General Assembly, acting after a Soviet undersecretary had stymied one investigation, set up another experts' panel that will report directly to the secretary general. The cause needs help. Soviet chemical warfare goes on.