Defense Secretary Caspar W. Weinberger went to the Senate yesterday to defend the administration's embattled proposal to sell sophisticated radar planes to Saudi Arabia, but the reaction only underscored that the $8.5 billion aircraft sale is in serious danger of a congressional veto.

Weinberger's testimony before the Armed Services Committee was supposed to focus on the military and technical aspects of the Airborne Warning and Control System (AWACS) deal. However, the real nature of the problem confronting the administration was made clear in an exchange between the secretary and Sen. Dan Quayle (R-Ind.).

Declaring that it is "an exercise in futility" to discuss the sales package in its present form, Quayle bluntly asserted that the deal has no chance of Senate approval unless modified to provide greater U.S. operational control over the AWACS equipment.

"What do we need to do to convince the administration that this package is not going to go through as it currently is?" Quayle asked.

When Weinberger replied that he was not prepared to discuss possible compromises at this time, the senator shot back: "You have no alternative . . . . If you don't want to believe me, ask the other senators."

The sales proposal, to be presented formally to Congress tomorrow, can be blocked if both houses vote against it. There is an apparently irreversible opposition majority in the House, and the Senate also seems certain to vote against it unless the administration revises the deal to include U.S. personnel in crews manning surveillance equipment on all AWACS flights.

The administration is working frantically behind the scenes to find a compromise able to satisfy a majority of senators. But The Washington Post, in a dispatch from Saudi Arabia published yesterday, quoted an unidentified member of the Saudi royal family as saying that U.S. crewmen would not be allowed in the AWACS planes after a necessary training period.

Presidential counselor Edwin Meese III was asked about The Post report while flying to New Orleans yesterday with President Reagan. He replied, "We're still talking. There's still a lot of talking going on."

Since the compromise negotiations began late last week, administration sources have insisted that, despite the hostile reaction from Saudi Arabia, the Saudi government has agreed to reserve judgment until the United States comes up with a detailed proposal for its consideration.

Earlier yesterday, as Reagan left the White House for New Orleans, reporters asked if a compromise was still possible. He replied, "I'm sure. We're still talking. There's still a lot of talking going on."

Meese also was asked about reports that the White House had downgraded the role of national security affairs adviser Richard V. Allen as its point man in the AWACS selling campaign.

He replied that Allen, who handled last week's flurry of compromise negotiations, is continuing as coordinator of administration strategy but that the primary role of dealing with the Saudis was returned to the State Department over the weekend.

In response to further questions about whether that was due to objections from Secretary of State Alexander M. Haig Jr., Meese said: "You know I never talk about personalities."

The Senate testimony by Weinberger and Gen. David C. Jones, chairman of the Joint Chiefs of Staff, was largely a restatement of the administration's familiar arguments for the sale--that it is necessary to maintain close relations with Saudi Arabia and protect the vital oil resources of the Persian Gulf and that it poses no threat to Israel's security.

However, questioning by most committee members made clear that many senators in both parties retain strong doubts about whether the presently proposed safeguard arrangements are sufficient to protect the AWACS planes against misuse or capture by forces hostile to the United States.

To back the argument that the planes are no threat to Israel, Pentagon officials put on a slide show emphasizing the limitations of the stripped-down version of the AWACS planes intended for Saudi Arabia.

That caused Sen. Henry M. Jackson (D-Wash.) to recall the glowing claims made for AWACS planes when the Pentagon was seeking funds for their development. He added caustically: "From the briefing, I get the impression that this is a pile of junk."