President Reagan's proposed $8.5 billion aircraft sale to Saudi Arabia won the support of one senator but incurred the opposition of another yesterday, as Senate Republican sources warned that Reagan may have to seek new concessions from the Saudis if he is to save the deal.

Several Republican senators and aides who asked not to be identified said that Reagan's lobbying campaign is faltering. They added that if he is to pick up the votes necessary to prevent a congressional veto of the sale, he must seek Saudi approval for greater U.S. control over the Airborne Warning and Control System (AWACS) planes when he meets today with Crown Prince Fahd in Cancun, Mexico.

These warnings came as Sen. Ernest F. Hollings (D-S.C.) declared that he will vote against the sale, while Sen. Warren B. Rudman (R-N.H.) said he will support the president. Both decisions had been expected.

However, it has become increasingly apparent on Capitol Hill that Reagan's intensive lobbying has not overcome the concern of many senators that the deal, as currently proposed, does not contain sufficient safeguards against misuse of the AWACS planes and other aircraft equipment.

Hollings' declaration brought back to 50--one half of the Senate--the number of senators publicly on record as intending to vote for a resolution disapproving the sale when it is considered by the Senate Wednesday. The resolution originally had 50 sponsors, but the White House subsequently persuaded three of them to switch sides.

In addition to the 50 now opposing the sale, three to six others are believed to be leaning against it. Thus far, the administration has lined up only 41 firm commitments of support.

The administration has been working on the assumption that several undecided senators and up to five or six sponsors of the resolution, chiefly freshman Republicans and southern Democrats, can be won over to its side. However, those it has targeted have remained firm in their opposition, and several have said they will change only if the Saudis agree to a system of joint control over the AWACS equipment.

In the last two days, two Democrats known to be high on the White House's target list--David L. Boren of Oklahoma, a sponsor of the resolution of disapproval, and Walter D. Huddleston of Kentucky, who is uncommitted--have called for Reagan to seek Fahd's agreement to joint control.

In the past, the Saudis have refused to consider that idea on the grounds it would be an infringement of their sovereignty. Although Reagan and Fahd are scheduled to meet today during the Cancun economic summit, administration officials have said there are no more concessions to be won and that no "bombshells" should be expected from the meeting.

Sen. Alan Cranston (D-Calif.), a leader of the opponents, predicted yesterday that the sale will be defeated handily, even if the Saudis make concessions. Cranston said:

"It appears to me that the opposition is becoming so firm that I no longer believe the sale would be saved by a Saudi compromise on joint crewing and control . . . . It now appears to me that it will be defeated not by a narrow margin, but by a significant one."

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) continued to insist yesterday that the opponents don't have a firm 50 votes and added: "It is not dead. It is very much alive, and you mustn't write it off yet."

However, sources close to the Republican Senate leadership said Baker privately concedes that the administration is way behind and now would be satisfied to see a 50-to-50 vote. In such a deadlock, Vice President Bush would be able to cast the deciding vote in favor of the administration.

The House last week voted 301 to 111 against the sale. If the Senate follows the House's lead, it would be the first time that Congress has exercised its prerogative to block a major arms sale.