President Carter is to begin a highly important series of meetings today with the leaders of Saudi Arabia, a potent force in the Arab world and keeper of the "oil weapon, " at a moment of uncertainty about the future of peace negotiations in the Middle East.

Crown Prince Fahd, the No 2 man in the Saudi hierarchy and the operating chief in many recent decisions, arrived at Andrews Air Force Base yesterday accompanied by senior officials including the oil minister, Zaki Yamani.

The meetings to begin today are the last in a series between Carter and top Arab leaders and, in some respects, the most interesting and important. The backdrop for the sessions is the unexpected election victory in Israel a week ago today of the rightistLikud Party headed by Menachem Begin, whose public statements have cast doubt on any chance for a compromise agreement with the Arabs. Begin's sudden illness early yesterday adds to the uncertainty.

Fresh from meetings in the Saudi capital with Egyption President Anwar Sadat and Syrian President Hafez Assad, Fahd is belived to be carrying a unified Arab position to Carter. Though the details are not known, the essence of the Arab view is that collapse of the U.S. backed peace initiative would turn the Middle East region toward new military conflict - and away from alliances with teh United States.

Egyptian Foreign Minister Ismail Fahmi told reporters Sunday that the "oil weapon" will be used automatically if Israel's new leadership refuses to surrender occupied Arab territory. Fahmi did not specify how oil supplies would be affected, but his remarks suggested the possibility of a new embargo.

The 1973 oil embargo by Saudi Arabia and other Arab states against the United States was lifted because of the U.S. promise to promote a Middle East settlement on terms acceptable to the Arabs.

In an interview published last weekend in a Beirut newspaper, Fahd declined to brandish the threat of a new oul emabrgo if peace negotiations collapse. However, he called for the United States to "throw all its weight" into the process of reaching at Mideast settlement based on Israeli withdrawal from occupied territories and creation of a Palestinian state. Fahd was quoted as calling this s Saudi "demand" on the U.S. in return for its cooperation with Carter's energy program.

Carter, in an address at Notre Dame University on Sunday, declared that failure to move now toward an Arab Israeli agreement could bring "disaster" for the international political and economic order as well as the Mideast region.

In what appeared to be a warning to Begin, Carter said U.S. policy would not be affected by changes of political leadership and that the United States "expects" Israel as well as her neighbors to continue to be bound by United Nations Resolutions 242 and 338. The resolution commit Israel to give back captured territories in return for peace - something which Begin insists that Israel will not do.

Saudi Arabia, with a mere 7 million people but about one-fourth of the oil reserves of the non-Communist would, is of great importance to the United States.

In the past few years the United States has become dependent in major fashion on imported oil as a source of energy, and Saudi Arabia has become its No. 1 supplier. Moreover, only Saudi Arabia has the potential for vast increases in future supplies for a growing U.S. economy.

In addition to the discussion of Mideast peace which is the main reason for the Fahd visit, Carter is likely to talk to the Saudi leadership about future oil prices - an area where the Saudis have dramatically demonstrated their mastery since last December - an d about future supplies to fuel U.S. cars, homes and industry and create a planned U.S. oil reserve of 1 billion barrels.

If Saudi Arabia should fail to raise its petroleum production to meet rising U.S. and world demand, oil prices would soar. Carter is reportedly prepared to ask for continued expansion in Saudi production capability.

The United States may also discuss the Saudi contribution to a multibillion-dollar internationl loan fund for financially pressed nations. The indications are that the Saudi pledge will be well below the $4 billion that major industrial nations hoped it would provide.

Reports from Cairo yesterday said U.S. Ambassador Hermann Eilts gave Foreign Minister Fahmi a letter from Carter conveying the Presidents assurances that the United States will continue to pursue the peacemaking process and still hopes to convene a Geneva conference on the Mideast sometime this year.