Diplomatic observers predict a continuing erosion of American influence in the Middle East if President Reagan fails to turn Congress around on the sale of advanced arms to Saudi Arabia. With Mr. Reagan on the verge of vetoing congressional rejection of the sale, administration and independent experts cite these potential consequences of a rebuff to the Saudis:
American capacity to influence Saudi Arabia will be diminished.
It will send a political signal to Iran that the United States is uninterested in Saudi Arabia and in the shipping of oil out of the Gulf. Iran has stepped up its attacks on Saudi ships of late, US officials say.
It will signal the Saudis that the US has loosened its support for their declared policy of helping Kuwait in the event of an Iranian attack. The smaller Gulf states will feel that they cannot rely on the United States.
With US contacts in the area loosened, moderate Arab states will increasingly have to propitiate the radical regimes.
Middle East experts say a snub of the Saudis potentially increases the power of Syria and the Soviet Union in the region. Despite Syria's support for the Iran-Iraq war and its opposition to Palestine Liberation Organization leader Yasser Arafat, Jordan is already moving to improve its ties with Damascus.
``Obviously the moderates won't all turn to the Soviet Union,'' says Talcott Seelye, former US ambassador to Syria. ``But Jordan already is buying Soviet arms and pushing into Syria.''
The sale of $354 million worth of shoulder-fired Stinger antiaircraft missiles, air-to-air Sidewinder missiles, and other sophisticated weaponry is relatively modest in magnitude. But it has become a touchstone of the President's ability to conduct Middle East policy. Reagan has had to pull back on other Mideast arms sales, including to Jordan, because of congressional opposition.
Having done little to win congressional support for the Saudi sale, Reagan is now pulling out all stops to ensure a veto is sustained. As a part of his campaign, he met this week with Senate leaders, who told him it would be an uphill fight.