A CONSENSUS in Congress holds that the trade agreement President Bush signed with President Gorbachev should not go into effect until, as House Speaker Tom Foley put it, there is ''some positive movement'' by the Soviet Union to let Lithuania take its independence. Mr. Bush has not made this a formal condition -- an omission for which he has come under fire -- but he acknowledges that Congress is insisting on the point. So should he. The moment when President Gorbachev is choking Lithuania with an economic embargo is not the right moment to remove restrictions on Soviet trade.

No one should take this position, however, without acknowledging its large implications. It comes at a possible price. The key tariff and credit penalties that stand to be lifted in a trade agreement are those imposed in 1974 to ensure Soviet Jewish emigration. As it happens, President Gorbachev has just hinted that his emigration policy, which has been generous, may now be clipped to meet Arab objections to a possible flow of Soviet Jews to the Israeli-occupied West Bank. That would put the trade agreement in the deep freeze and severely set back Soviet-American relations on the political level. Presumably Mr. Gorbachev will find a way to back off this ill-considered course.

Assuming that he does, Congress would be left having added to the trade law's original specified standard of free emigration a major new standard, not inscribed in law, of Lithuanian independence. This has its reasons, but would take its own toll of the standard of constancy expected of a serious government.

There is more. The Soviet Union, a country of 15 republics and more than 100 ''nationalities,'' faces ethnic unrest and claims for ethnic autonomy or statehood without end. Are economic ties with Moscow to be hinged on the satisfaction of the many ''Lithuanias'' that may line up at the door of Congress? Do not hastily dismiss the thought. As a multi-ethnic democratic society, the United States is vulnerable to continuing pressures to mortgage its commerce to Soviet ethnic aspirations, not to speak of democratic ones. As a society committed to international economic exchange, it is also vulnerable to continuing pressures to make economic decisions -- on grain exports to the Soviet Union, for instance -- mainly on economic grounds. Lithuania takes Americans to the heart of an issue they will be grappling with for a long time.