NO SINGLE PIECE of congressional intervention in foreign policy has been more well-meaning in intention and more mischievous, damaging and even tragic in results than the Jackson-Vanik amendment. Written in 1974 with the Soviet Union and Jewish emigration specifically in mind, it requires non-market (communist) countries to permit free emigration in order to receive normal treatment under American tariff laws. The Soviet Union saw the amendment as a hoop of humiliation and refused to jump through it. The result: trade possibilities collapsed and emigration collapsed.

The matter is relevant now because a Jackson- Vanik denial of normal tariffs looms over Romania. Romania acquired "most-favored-nation" (MFN) status, as normal tariff status is misleadingly called, in 1975 in a nice economic marriage of political convenience. Communist and a Soviet ally but still highly nationalistic, Romania wanted MFN to bolster its independence from Moscow. The United States, including the Reagan administration, has long pursued a policy of differentiating East European nations according to their openness to Western ties. Even before 1975, Romania had allowed Jews to emigrate in large numbers, mostly to Israel. After 1975, general emigration continued: in 1982, 1,800 went to Israel, 12,700 to West Germany, 4,000 to the United States.

Last year, however, being deep in debt to the West, Romania started talking of making emigrants pay, in hard currency, the "debt" they owed for state-provided higher education. Romania sees itself as an undeveloped country unable to afford a continuing "brain drain." American officials, advised by their lawyers and Congress that these "debts" were nothing other than the education "tax" Jackson-Vanik specifically bars, quietly warned Bucharest. If the "tax" were imposed, they would have to invoke Jackson-Vanik. But the Romanians, who make a policy of standing up to Moscow, were not about to yield to Washington.

The tax is now on, and unless something unexpected happens the tariffs on Romanian goods will shoot up on June 30. The estimated trade loss over the next five years is put at $2 billion. What happens to Romania's capacity to stay out of Soviet clutches, and to its and others' readiness to deal with the United States, remains to be seen.

So here is the situation: a law passed to spur emigration from Russia halted most emigration from that country and had major political side effects. The same law is now working to punish Romania. Romania has in fact done something perverse--to cope with a foreign exchange crisis. But it has in many respects an outstanding emigration record, though its procedures are sticky and its human rights record in other respects--such as treatment of Christians, a matter unmentioned in Jackson- Vanik--is terrible. Romania is, moreover, a country whose independence from Moscow and access to both sides in many of the world's feuds has made it a valuable international citizen from the American point of view.

That leaves Jackson-Vanik now to be applied only to Hungary, which sends virtually no emigrants to the United States, and to China, whose size makes emigration guarantees a joke.

Is it not past time to amend this law?