President Reagan opened his campaign to win one of the toughest fights awaiting him in Congress this fall by reaffirming yesterday his intention to sell sophisticated AWACS radar reconnaissance planes to Saudi Arabia.

The president called this sale an essential element of his Middle East policy and asked members of Congress not to make up their minds about the sale until they have heard his arguments supporting it.

A majority of senators have indicated they will vote against the sale, which is strongly opposed by Israel. Reagan, in a letter sent Tuesday to the Republican and Democratic leaders of both houses and made public yesterday, said he has heard that many legislators have "been under some pressure to take an early position against" the sale.

"I hope no one will prejudge our proposal before it is presented," he said.

The letter was sent to Egyptian President Anwar Sadat arrived in Washington for his first meeting with Reagan. White House deputy press secretary Larry Speakes denied that the letter was timed to Sadat's visit, saying it was sent now to reach members of Congress before they leave for the August recess.

"We will make a strong case to the Congress that [the sale] is in the interest of our country, the Western alliance and stability in the Middle East," Reagan said in asking leaders to urge legislators to await the administration's formal presentation of the sale before making up their minds.

The letter was sent to Senate Majority Leader Howard H. Baker Jr. (Tenn.), Senate Democratic leader Robert C. Byrd (W. Va.), House Speaker Thomas P. (Tip) O'Neill Jr. (Mass.) and House Republican leader Robert H. Michel (Ill.).

The sale has aroused strong opposition among many members of Congress who usually support Reagan. In the House, Rep. Jack Kemp (R-N.Y.) organized a colloquy in which most speakers assailed the proposed sale. In the Senate, several conservative Republicans who do not have large Jewish constituencies have expressed concern about it.

As well as the AWACS planes, which could monitor aircraft movement inside Israel, the sale includes equipment to enhance the performance of F15 fighter planes already sent to the Saudis.

Administration officials have said the AWACS sale is necessary to restore U.S.-Saudi relations frayed during the Carter administration. The Reagan administration has made a point of publicly praising Saudi Arabia for assisting in efforts for a cease-fire in Lebanon.

Opponents argue that the sale unnecessarily endangers Israel and point out that Saudi Arabia is a major financial backer of the Palestine Liberation Organization. They say that the Saudi government is fragile and that, if the royal family there is overthrown, the AWACS planes could fall into the hands of less moderate forces.

U.S. military personnel and civilians would play a major role in the Saudi AWACS program for the life of the planes, senior Reagan aides have said. The presence of Americans is aimed, in part, at allaying fears that the planes could be used against Israel.