The political as well as strategic relationship between the superpowers hangs in the balance with the ratification of the SALT II treaty, according to Georgi Arbatov, the Soviet Union's most prominent expert on American affairs.

Arbatov, director of Moscow's Institute of the U.S.A. and Canada, said in an interview that Senate rejection of the strategic arms limitation treaty (SALT) would make it impossible to sustain a normal political dialogue between the two nations. Defeat of the treaty after seven years of negotiation under three U.S. presidents, he said, would lead to the conclusion in Moscow that "you simply cannot have serious business with the United States."

Arbatov, the leading Soviet equivalent of an American "Kremlinologist," is a member of the Supreme Soviet and of its Foreign Affairs Committee, and an alternate member of the Central Committee of the Soviet Communist Party. On a month-long visit to the United States, Arbatov left Washington yesterday for a week's tour of southeastern states, from a South Carolina plantation to an Alabama poultry center, under the sponsorship of the Southern Center for International Studies.

Despite his dark portrayal of the political consequences of SALT's rejection, Arbatov also said the mutual stakes of the two nations for sheer survival are so great that there would be, in the long run, no alternative to a renewal of dialogue. However, he said this might take years or even longer after a serious falling out. He foresaw the possibility in the meantime of an unrestrained arms race and a reassertion of cold war antagonisms on a scale that could gravely endanger both nations.

Arbatov, like other Soviet leadership figures and the press and propaganda organs, strongly backed the SALT II treaty as a bargain in the security interest of both nations. Reacting to some statements here, 'however, he declared that "it is an inaccurate notion that we are more interested in SALT than you are."

The belief that the Soviet commitment to the SALT treaty is over-whelming, as he noted, gives rise to the expectation that the Russians may be willing to display particular restraint during the delicate period of the ratification debate. Arbatov said that while the Senate deliberations, like all other political facts, must be taken into account, "I don't think you can expect special concessions in our policy because of it."

He made the comment in response to a question about Soviet restraint in the deepening dispute in southern Africa and the pending Soviet decision about continuation of the United Nations force in the Sinai.

Arbatov's view, like others coming out of Moscow, was "negative, of course," about the possibility of accepting senatorial amendments to the treaty. "We've made all the compromises we consider to be possible and admissible" during the course of the lengthy negotiations, he said. He compared the Soviet reaction to amendments to that of the United States if the Supreme Soviet, at this point, demanded additional "concessions."

Turning to the positive effects of the strategic arms treaty, Arbatov said that Soviet-American relations are "at a crossroads" and expressed the hope that the completion of SALT II will lead to improvements on a broad front. Beyond its strategic merits the treaty "also has a tremendous political meaning," he said.

The Soviet expert made it clear that Moscow is hoping for at least a partial renewal of the comfortable political atmosphere of detente generated by the Nixon administration around the SALT I agreements in 1972. He rejected the possibility, inherent in some statements by high Carter administration officials, of a SALT II accord that is accepted strictly on strategic grounds, in effect, "SALT with detente."

"This whole concept is wrong - arms control is not an end in itself . . . We have to care about many problems in common," Arbatov said.

On the subject of U.S. relations with the People's Republic of China, a subject of great concern in Moscow, Arbatov seemed notably more relaxed than he and other Soviet figures were just after the Washington-Peking rapprochment at the end of last year. In his opinion, American euphoria over relations with China "is on the wane" because of the Chinese invasion of Vietnam and lowered expectations of the potential economc benefits of trade with China.

"The more you get acquainted with China and its problems," said Arbatov, "the more realistic you will become."

He added that "there is a place for trade, for normalizing relations, but it will not bring you the solution to major problems. And any attempts to play China against the Soviet Union can become very dangerous and unproductive." CAPTION: Picture, GEORGI ARBATOV . . . member of Supreme Soviet