President Carter yesterday stepped up his personal lobbying for his proposed Middle East warplanes sale, arguing that rejection by Congress would be a "breach of trust" with Egyptian President Anwar Sadat and a severe blow to the prospects for peace in the Middle East.

"The choice is stark and fundamental," the president said in a letter delivered to every member of the Senate yesterday afternoon.

"Shall we support and give confidence to those in the Middle East who work for moderation and peace? Or shall we turn them aside, shattering their confidence in us and serving the cause of radicalism?"

The letter, which was also delivered to House Speaker Thomas P. (Tip) O'Neill Jr. (D. Mass.) and the ranking members of the House International Relations Committee, particularly emphasized the stakes in the arms sale for Sadat, whose dramatic peace initiative last year had a major impact on American public opinion.

The administration's $5 billion warplane package would provide 50 F5F, fighter-bombers to Egypt, the first American planes to be supplied to the Arab country. It would also provide 75 F16 fighter-bombers and 15 F15 fighters to Israel and 60 F15s to Saudi Arabia.

The Senate Foreign Relations Committee deadlocked 8 to 8 Thursday over a resolution to disapprove the sales. The full Senate is to debate the issue and vote on it Monday.

The sales will go through unless both the Senate and the House pass resolutions disapproving them.

"It is my considered judgement that the aircraft sales to Egypt are essential to enable President Sadat to continue his efforts for peace," Carter said in the letter.

"At great personal and political risk, President Sadat has taken an initiative which has created the best prospects for peace in the Middle East in three decades," he continued. "With similar risks, he has turned away from a relationship with the Soviet Union and placed his trust in the United States.

"To reject the proposal aircraft sale to Egypt would be a breach of that trust."

"The president struck the same theme earlier yesterday in an interview with newspaper editors. Rejection of the sale to Egypt, he said, would be "a terrible blow" to Sadat and to U.S. relations with Egypt.

But while Carter, for tactical reasons, stressed Sadat's new ties with the United States, the key to approval of all the sales is the 60 F15s earmarked for Saudi Arabia, a proposed sale that is adamantly opposed by Israel and its supporters in Congress.

The Saudis are the United States' major supplier of imported oil, a factor that the president referred to in his letter only indirectly.

Contending that the plane sale gives the United States a chance to "enhance its relationship" with the Saudis, Carter said.

"I must tell you with great gravity that it is an opportunity that we will quickly lose if we do not grasp it immediately. If the Saudis are forced to turn elsewhere to meet their defense needs, it will unquestionably impair the peace process. Moreover, the erosion of confidence will inevitably have a far broader - and adverse - impace on the wide range of issues on which we have been working in close harmony."

Carter also argued that Israel stands to gain by the proposed warplane sales to its Arab neighbors.

"It wouls be against Israel's interest if moderate nations are brushed aside by the United States, opening vast possibilities for the intrusion of hostile influences" in the Middle East, he said.

White House press secretary Jody Powell said the president has also been arguing his case in a series of telephone calls to senators. Powell repeated his prediction that the administration will prevail in the Senate vote Monday.