Washington — 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.
After the meeting, Sen. Richard G. Lugar (R) of Indiana, chairman of the Senate Foreign Relations Committee, said the President would meet with American Jewish leaders to seek support for the sale. AIPAC, a key pro-Israel lobbying group, has not vigorously worked the Congress to bar the sale, leading to speculation that it is more interested in stopping the delivery of AWACS early-warning aircraft to Saudi Arabia this summer.
In a clear effort to help swing the vote in the Senate, where lawmakers in an election year are reluctant to support policies not favored by Israel, Mr. Lugar is calling on the American Jewish community to weigh in on the President's side.
``It is important for those who are strongly pro-Israel to come to the aid of the President in this situation,'' he told reporters. ``Because if he is hobbled, . . . Israel is going to suffer. . . . For the President to deal effectively in the Middle East he has to be able to work with the moderate states.''
The Senate recently rejected the arms sale, 73 to 22. The next day the House turned it down, 356 to 62. Reagan will concentrate on the Senate, where he must persuade six or seven senators to change their mind.
Some diplomatic experts voice concern about what they see as a White House strategy of relying on personal support for the President to sway the senators.
``If you're serious about long-term policy, you shouldn't win it by doing the President a favor, but on grounds that it serves some national interest,'' says William Quandt of the Brookings Institution. In the end, Mr. Quandt says, the President may get his way, but ``this won't . . . make a dent in public understanding.''
Another consequence of a defeat on the proposed sale is that Saudi Arabia would turn to other Western suppliers for arms, costing the US jobs and business. Last year, after the President was forced to withdraw the sale of additional advanced F-15 aircraft to the Saudis because of congressional opposition, the Saudis purchased British Tornadoes instead.
Administration officials say they are troubled by the rise of anti-Arab feeling both in Congress and among the public as a result of the terrorism issue. Lawmakers, they say, are not making a distinction between the moderate Arab nations like Saudi Arabia and the radical states, such as Libya.
``To lump them is to foster the radicalism that Congress professes to be against,'' a US official says.