The Soviet Union, in the person of President Leonid Brezhnev, is escalating efforts to counter the new US diplomatic initiative in the Mideast.
Israel's further military move Sept. 16 into west Beirut could aid the Soviets' cause, some diplomats here argue, by prompting fresh Arab doubts over the Americans' ability to influence Israeli policy.
The Soviets have been signaling concern that President Reagan's recently toughened public position toward Israel - despite his parallel opposition to establishment of a Palestinian state - might lure key Arab leaders and thus further trim Moscow's influence in the region.
Against this background, President Brezhnev, in a speech late Sept. 15, reassembled past Soviet policy positions in a package of six principles for a Kremlin-supported peace in the Middle East.
Mr. Brezhnev used the speech to contrast Soviet backing for an eventual Palestinian state, and for the Palestine Liberation Organization, with continued American opposition on both counts.
He argued that the Soviet approach was ''not at variance'' with a compromise Arab negotiating plan worked out at a recent summit in Morocco, while the Reagan administration initiative amounted to mere ''pretending to find a settlement'' that would, in fact, deny key Arab demands.
But the Soviet President seemed to be aiming less at outright Arab backing for his approach than at complicating US policy and in furthering a longtime Soviet bid for a piece of the Arab-Israeli diplomatic action.
In pursuit of the latter goal, Mr. Brezhnev included explicit support for the secure existence of Israel as part of a peace settlement. This was, indeed, ''at variance'' with the Arab summit declaration.
In pursuit of the former, he came close to backtracking on past Soviet support for the existence of Israel by saying: ''In opposing the creation by the Palestinians of their own state, the Washington administration is thus also calling into question the legal basis of Israel's existence.'' Foreign diplomats here assume, in absence of further indication to the contrary, that this statement was intended as a propaganda sally, not as foreshadowing a formal retraction of Soviet acceptance of Israel's existence, a move seen sure to complicate Kremlin efforts for eventual reentry into Arab-Israeli diplomacy.
The Soviet President left open the question of how his peace principles would be implemented and played down a February 1981 call for an international peace conference that has yet to make headway. He thus seemed to suggest that further Soviet moves would depend largely on what the Arabs, and Americans, do next.
Since the Reagan administration seems intent on keeping Soviet influence in the Mideast to the absolute feasible minimum, the Arabs will be the key players in the immediate future.
As expected, the first indication of Arab response stopped short of fully endorsing the announced Soviet approach. The reply came in a communique capping the visit here by the President of South Yemen, the only Marxist state in the Arab world and the Soviets' most reliable ally there. Mr. Brezhnev made his Mideast policy sttement at a banquet for the Yemeni President.
Though a Soviet ally, the Yemeni is among the harder-line of Arab leaders on recognition of Israel's right to exist. Thus the joint communique said only that a Mideast settlement must be sought through ''collective efforts . . . including the PLO,'' without committing the Yemeni President to the principles Mr. Brezhnev linked to such a settlement.
The Soviets, the communique said, had pointed out ''that the basic provisions of the Arab (summit) plan for a Mideast settlement . . . coincided with what the Soviet Union has been seeking for many years.''
Of potentially greater importance to the Kremlin will be the response of Jordan's King Hussein - who, under Reagan's initiative, would be politically linked with a system of ''Palestinian self-government'' stopping short of a full-fledged Palestinian state - and of PLO chairman Yasser Arafat.
King Hussein, increasingly unhappy with US policy in recent years and on record as favoring some Soviet role in the negotiating process, had words of praise for the new Reagan approach in a British television interview Sept. 13.