Whether to grant the Soviet Union most-favored-nation trade status will be the next legislative battle to determine the course of U.S. Soviet trade relations.

At issue is the 1974 Jackson-Vanik amendment to the trade act that forbids most-favored-nation status for the Soviet Union until the Kremlin gives "assurances" that it will relax emigration policies for Jews who wish to leave.

But just what constitutes an "assurance," and whether the Soviets have already demonstrated a commitment to freer emigration, have split the administration, the powerful Jewish lobby, and even the legislation's co-sponsors, Sen Henry M. Jackson (D-Wash.) and Rep. Charles A. Vanik (D-Ohio).

The number of Jews leaving the Soviet Union this year should reach 50,000, the highest in history. Also, other recent Soviet actions such as the release of five dissidents in a prisoner exchange last April are seen as Soviet concessions to the United States, to rid Moscow of the trade restrictions.

Jackson would like to continue the trade restrictions until the Soviets give "a firm commitment to freer emigration," allow people to leave at will and free all remaining political prisoners.

Vanik, on the other hand, believes that "the facts constitute assurances," and that the Soviets should be rewarded for their recent actions by the granting of a presidential waiver of the trade restrictions.

The law provides for a presidential waiver of up to one year at a time, subject to congressional approval - what Vanik calls the "carrot and stick" characteristic of Jackson-Vanik, That is, reward the recent Soviet moves by lifting the restriction - supplying a carrot - and if Moscow does not continue the trend of freer emigration after a year's time, slap the restriction back on - applying a stick.

Jackson and Vanik took their differing interpretations before the National Conference on Soviet Jewry in Starkly contrasting speeches to the group's annual policy meeting here in Washington. On Sunday night, Jackson called the waiver "a leap in the dark." Yesterday, Vanik told the same gathering "if we take the hard line, there are inherent risks."

The national conference apparently agrees with Jackson and is expected to recommend later this week that the Carter adminstration withhold the waiver until the Soviets release all political prisoners, allow those who have been refused exit visas to leave, and change their procedures to make emigration easier and faster.

The administration has been looking for a way to grant most-favored-nation status to the Soviets for some time now.

Administration officials hope Carter can come away from this week's Vienna summit meeting with at least some implicit understanding from Soviet President Leonid I. Brezhnev.

But those same officials say there is no way possible to get the kind of concrete assurances that would please the so-called hardliners, much less get the Soviets to release all their internal political prisoners.

The Soviet restriction is a particular nuisance for the administration because China is apparently ready to receive most-favored-nation trade status. The official line is that the United States should pursue a policy of "evenhandedness," which means the administration does not want to grant most-favored-nation status to the Chinese without granting it to the Soviets as well.

Critics of the waiver dismiss the "evenhandedness" concept as a non-is-sue.