President Reagan's fight to save his proposed $8.5 billion aircraft sale to Saudi Arabia produced a dramatic breakthrough yesterday as 10 senators openly threw their support to the president and put him in striking distance of victory when the Senate votes on the package today.

The backing mustered by Reagan, including a startling switch by Sen. Roger W. Jepsen (R-Iowa), who had been one of the most outspoken opponents of the sale, raised the possibility that the vote will end in at least a 50-50 tie. That would leave the opposition one short of the majority required to pass a resolution disapproving the sale if all 100 members vote.

Adoption of the resolution requires a majority of those senators present and voting. Additional senators could swing toward the administration and give it a majority, but the White House's strategy appeared last night to center on producing a tie vote of the full Senate.

Its chances of getting a tie, or even a majority, were acknowledged last night by one of the opposition leaders, Sen. Alan Cranston (D-Calif.). He said: "The odds have shifted. The advantage now lies with the president."

The administration's hope of a tie hinges on the expected, but still undeclared, defection from the opposition of two other Republican senators, Slade Gorton of Washington and Mark Andrews of North Dakota. If they switch into Reagan's column, the opposition's only chance to defeat the sale will depend on Sen. Russell B. Long (D-La.), the one senator who has not taken a stand.

Most congressional sources have been expecting Long to vote for the sale. However, he would only say yesterday that he plans to make his decision after listening to the day-long debate leading up to the vote at 5 p.m. today, and there was no immediate sign of how the veteran Louisiana senator will react to being cast in the unexpected role of determining whether the sale goes through.

When the debate opens this morning, the lineup, based on the stated intentions of various senators, will show 52 opposed to the sale, 47 supporting the president and one, Long, undecided. However, it now is widely thought in the Senate that Gorton and Andrews, both cosponsors of the disapproval resolution, will change sides when the roll is called.

That would make the vote against Reagan 50 to 49. It then would be up to Long to determine whether the final tally is 50 to 50, which would defeat the disapproval resolution, or 51 to 49, which would give the opposition the edge they need to defeat the sale.

The president sounded hopeful about the vote as he left the White House early last night en route to Richmond to speak at a campaign rally in behalf of GOP gubernatorial nominee J. Marshall Coleman. Before boarding the helicopter on the South Lawn, Reagan said he was "cautiously optimistic" about today's vote.

He declined to speculate on the number of votes he still needs for the sale to go through, but said it was "much closer than has been reported in the last few days." He said he expects more senators to "jump on or climb on" before the vote, but insisted that he had not made any deals to win support. "I don't make deals," he said.

The president said he has never argued that senators who do not support the sale would weaken his hand in conducting foreign policy, "although that would be a natural assumption to make."

During the helicopter flight to Richmond, Sen. John W. Warner (R-Va.) talked with the president. He later told reporters he had told Reagan that the one thing that was on senators' minds was whether the president had made any promises to get votes for the package. "I looked the president square in the eye and asked him if he gave away anything," Warner said, "and his direct answer was absolutely not."

Reagan also plugged for the AWACS vote in his Richmond speech, but his tone was less optimistic.

He said he was "deeply concerned" about the outcome. "Not about whether the Saudis actually get the airplanes, because they'll get such aircraft, whether we provide them or not. What concerns me is how a rejection by the Senate will affect peace in the Middle East, and what it will do to our ability to provide the leadership so necessary for the security of our nation. We need the goodwill of the Saudis. They provide us with a significant part of our oil, yes, but more than that, they represent a moderate force in the Middle East. Rejection tomorrow, I am afraid, would be a step toward closing them out of any peace initiative."

Sounding as if he were rehearsing for a scheduled debate with Sen. Bob Packwood (R-Ore.) on the "Today" show (NBC, WRC) this morning, Reagan said the vote "is as important to the ultimate security of Israel as it is to our own interests." He was applauded when he said: "I would never risk the security of Israel."

Mideast peace can only come, Reagan said, "by drawing the moderate Arab states into the peace-making process, along with Egypt and Israel, who are already part of it."

At issue is the largest proposed arms deal in history, one involving the sale to the Saudis of five Airborne Warning and Control System (AWACS) radar surveillance planes and additional sophisticated aircraft equipment. The House voted 311 to 111 against the sale on Oct. 14; if the Senate follows its lead, it will be the first time that Congress has exercised its prerogative to veto a major arms deal.

The administration, aided by Senate Majority Leader Howard H. Baker Jr. (R-Tenn.), has waged an all-out lobbying blitz to reverse the majority originally leaning against the sale on the grounds that the equipment might be used against Israel or fall into the hands of forces hostile to the United States.

Yesterday the administration's determined chipping away finally began to show results as Jepsen switched sides publicly, Gorton and Andrews reportedly gave the White House their assurances they would follow suit and nine previously uncommitted senators announced their support of the sale.

The uncommitteds who came into Reagan's corner were David L. Boren (D-Okla.), Robert J. Dole (R-Kan.), James J. Exon (D-Neb.), Pete V. Domenici (R-N.M.), Walter D. Huddleston (D-Ky.), John Melcher (D-Mont.), Frank H. Murkowski (R-Alaska), Harrison H. Schmitt (R-N.M.) and Charles E. Grassley (R-Iowa).

Dole, Domenici and Schmitt had been expected to back the president. However, the opponents had been hopeful of winning some of the others, particularly Exon and Boren, as a cushion against anticipated defections from the list of those who had declared their intention to vote against the sale.

The opposition had expected that the intense pressures being exerted by the White House would pry away some freshman Republicans like Gorton and Andrews. But they appeared stunned by the change of heart announced yesterday by Jepsen, who had been one of the originators of the disapproval resolution and who was still publicly proclaiming his opposition to the sale less than a week ago.

Word of Jepsen's switch began circulating yesterday morning, and in the afternoon he appeared in the Senate press gallery with Grassley to make a joint announcement that they would back the president.

Jepsen said he was changing because he had been given "highly classified information" that had lessened his concern about possible misuse of the AWACS equipment. He also said the assassination of Egyptian President Anwar Sadat had "changed the stakes" and made clear that a defeat on AWACS would impair Reagan's ability to conduct foreign policy, adding that "large numbers of Iowans have been telling me that they support the president's decision. . . . "

The Iowa senator stuck to those arguments in the face of reporters quoting back to him his past condemnations of the proposed sale and his remark at the White House last week that Sadat's death only underscored the instability of the Arab governments in the Middle East.

Sources close to Jepsen later privately described him as "in anguish over his decision." The sources said he continues to think that the sale is ill-advised; but, they added, he changed his position out of a sense of loyalty to Reagan, who has supported him in his campaigns and efforts to retire his campaign debts.

This appeal for loyalty to the president and the Republican Party has been one of the main arguments used by the White House and Baker on the 18 Republicans who were among the original 49 cosponsors of the disapproval resolution.

Following the assassination of Sadat, two--Orrin G. Hatch of Utah and Alan K. Simpson of Wyoming--acceded to these appeals and switched. Now, Jepsen, and apparently Gorton and Andrews, have followed suit; and some administration sources predicted yesterday that one or two more GOP freshmen might be ready to change sides by the time the vote comes.

However, Cranston, while conceding that Gorton and Andrews appear to be lost, said he considers the 50 other senators who have declared their opposition to be "firm," and added that he believes the issue will be settled by Long's vote.

In seeking to woo individual senators, the White House made effective use of a still secret letter from Reagan spelling out the assurances that have been built into the deal as a means of preventing misuse of the AWACS and other equipment. The letter has been held back by the administration as a bargaining device that was used to address the specific concerns of senators who showed signs they might support the sale.

Murkowski said yesterday he had decided in favor of Reagan because language had been put into the letter to deal specifically with questions bothering him. Similarly, Gorton and Andrews are expected to justify their switches on the grounds that the letter has been changed to give them the assurances they sought.