President Reagan met with six more senators yesterday as part of a last-ditch lobbying blitz to rescue his embattled $8.5 billion aircraft sale to Saudi Arabia. By day's end, he had won the vote of one senator whose support had been expected, but lost another who had been in the uncommitted column.

Sen. Patrick J. Leahy (D-Vt.), who was not among the six, announced that he will oppose the president tomorrow when the Senate votes on a resolution to disapprove the sale. On the other side, Sen. William L. Armstrong (R-Colo.) said after seeing the president that he would support the sale. He had been counted by both sides as likely to back Reagan.

Of the other senators who met with Reagan yesterday, four, who are among the cosponsors of the disapproval resolution, said later that the president had not changed their minds. They are Republicans Mark Andrews of North Dakota, John C. Danforth of Missouri and Robert W. Kasten Jr. of Wisconsin, and Democrat Howard W. Cannon of Nevada.

A fifth declared opponent, Roger W. Jepsen (R-Iowa), declined Reagan's invitation to come to the White House.

Sen. Frank H. Murkowski (R-Alaska), who has been undecided, left the White House saying he still has not made up his mind.

The continued opposition of Andrews, Danforth, Kasten, Cannon and Jepsen, coupled with Leahy's declaration, put at 53 the number of senators who are on record as intending to vote against the sale of Airborne Warning and Control System (AWACS) planes and other equipment comprising the biggest proposed arms deal in history.

Adoption of the disapproval resolution requires a majority of those senators present and voting tomorrow. If all 100 members are present, 51 votes will be needed to put the resolution across. Two weeks ago, the House voted 301 to 111 to disapprove the sale; if the Senate does the same, it will mark the first time that Congress has vetoed a major arms deal.

Despite the evidence that Reagan's lobbying has not dented the apparent majority against the Saudi deal, the White House and its allies in the Senate Republican leadership continued to insist yesterday that they will turn sufficient votes around by tomorrow to enable the sale to continue. However, the administration persisted in refusing to say who it believes will switch.

White House deputy press spokesman Larry Speakes said Reagan plans to see as many as 12 senators today and tomorrow. But, in a reversal of the administration's past willingness to announce who had been invited to the White House, Speakes refused to reveal who the next lobbying targets will be.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) has been insisting that, despite the public record, several senators still have not made up their minds and can be expected to support the president at the last minute. Supporters of the sale also have hinted that some might agree to vote "present" and then change to a vote against the disapproval resolution if the initial count shows that their votes will give Reagan a majority.

Sen. Alan Cranston (D-Calif.), a leader of the opposition, said yesterday he now thinks there are 56 votes against the sale, 41 in favor and three undecided. Cranston and other opponents have been urging Reagan to withdraw the sales package, but Speakes said yesterday, "We expect to see it come to a vote."

Cranston also took aim at Reagan's statement Saturday, following his return from an economic summit in Mexico, that the opponents "are not doing a service to the country."

In a statement to the Senate, Cranston asserted: "It's not service to the country to support the president when he's wrong." Then, focusing on administration charges that Israel has interfered in domestic U.S. affairs by opposing the sale, Cranston added: "By accusing the Israeli government of interfering in American politics, by raising the specter of domestic anti-Semitism, the Reagan administration is doing a gross disservice . . . . The administration apparently is seeking to make a scapegoat of Israel and Israel's friends in the United States. The administration has no one to blame but itself for its problems with the Saudi arms package."

Speaker of the House Thomas P. (Tip) O'Neill Jr. (D-Mass.) also attacked Reagan's weekend assertion. He said, "The defense of the nation is never a political item. I know of no man who would vote politics when it comes to the defense of the nation."

O'Neill also suggested that Reagan pull back the package and try to renegotiate in a way that will make it more acceptable to Congress. "Maybe to save embarrassment to himself, he ought to withdraw it for a while," the speaker said.