President Reagan, anticipating that the House today will vote against his proposed sale of radar planes to Saudi Arabia, plans to send a new letter to the Senate in hopes of winning sufficient support there to prevent congressional veto of the embattled $8.5 billion deal.

Reagan's new effort to woo a majority of the Senate was revealed by Sen. Larry Pressler (R-S.D.), an opponent of the sale. After meeting with the president yesterday, he told reporters that the president's letter probably will go to the Senate before the Foreign Relations Committee votes Thursday on a resolution to disapprove the sale.

The deal will be blocked if both houses of Congress vote against it before the end of the month. Speaker of the House Thomas P. (Tip) O'Neill Jr. (D-Mass.) predicted yesterday that the resolution of disapproval will pass the House by a 3-to-1 margin today.

That will leave the Republican Senate the battleground for Reagan's efforts to save the sale of Airborne Warning and Control System (AWACS) planes and other advanced aircraft equipment. The president's 15-minute meeting with Pressler yesterday was part of an intense administration effort to change the positions of Republican senators who are leaning against the deal.

Pressler, a member of the Foreign Relations Committee, is among the cosponsors of the Senate disapproval resolution. Following the meeting yesterday, he told reporters, "I'm still in the 'no' column."

However, he indicated he might be willing to switch if Reagan can overcome his concerns about the AWACS equipment being used against Israel or falling into the hands of forces hostile to the United States. On Monday, Pressler proposed that the administration balance the deal by providing Israel with new radar and jamming equipment.

"I'm looking at all the possibilities, and I certainly want to help the president where I can," Pressler said. He added that he had discussed his proposal with Reagan and said: "There are certain criteria that I have, and if they develop, it's still possible I can help out."

The senator said the letter being prepared by the White House would attempt to deal with questions of American personnel in the AWACS crews and the control and safeguarding of the aircraft. The Saudis have rejected calls for joint control of the planes, and the Senate vote will hinge on whether Reagan can convince a majority that sufficient safeguards already are built in.

David Gergen, White House communications director, said later that the letter was still being drafted and probably would not be ready to go the Senate today.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) described the proposed letter as a statement of Reagan's commitment to the security of Israel. Baker also expressed optimism that the Senate will back the president: "I think the momentum is with us...I think there's a chance we can win this thing."

Former president Carter, who formally endorsed the sale on Monday, said yesterday he would not have offered the Saudis control over the AWACs planes, but added that good U.S. relations with Saudi Arabia and the Arab world now make it important to go through with the deal as a sign of U.S. reliability.

"I could make a good argument either way," Carter said. "But the point I make is that, once the president of the United States makes a commitment, then, if it is a close call, the Congress should support him...."

Sen. John H. Chaffee (R-R.I.) announced yesterday that he will vote to support the sale. Although Chaffee previously had said he was uncommitted, head counters on both sides had expected him to back the president.