The Reagan administration, on notice that its plan to sell sophisticated radar aircraft to Saudi Arabia is in serious danger of being rejected by Congress, yesterday denied growing speculation that it will seek Saudi concessions to make the deal more acceptable to opponents.

"It's not up to us," a senior administration official told reporters in response to questions about the possibility of switching from the proposed sale to a leasing arrangement that would let the United States retain joint command of the airborne warning and control system (AWACS) planes.

The official, who declined to be identified, cited Saudi Arabia's objections to such an arrangement as an infringement of its sovereignty, and echoed the contention of Secretary of State Alexander M. Haig Jr. before the Senate Foreign Relations Committee Thursday that the idea is "just not a practical solution."

Similarly, Frederick G. Dutton, a Washington attorney who is special counsel to the Saudi government, said his client would "take a defeat" of the entire $8.5 billion sales package rather than agree to a leasing plan. "It would be an unacceptable intrusion of their territory and sovereignty," Dutton said.

Despite the administration's insistence that it intends to fight for the package in its present form, congressional sources said there is a growing sense on Capitol Hill that if the administration wants to stave off defeat in the Senate, it will have to put together a proposal permitting some greater measure of U.S. control over the AWACS.

According to these sources, the question of control is emerging as central to whether many of the 50 senators, who Thursday co-sponsored a resolution to block the sale, can be induced to drop their opposition.

Many senators are anxious about keeping an American hand on the AWACS controls as an assurance that the technology involved will be protected properly and not be used by the Saudis as a threat to Israel's security.

The sale can be blocked if both houses of Congress vote against it before Oct. 30. There is an apparent strong majority against the deal in the Democratic House, and the administration has concentrated its lobbying on the presumably more sympathetic Republican Senate.

But, as Thursday's resolution showed, the administration faces a steep uphill battle to win the necessary Senate votes. Several of the 32 Democrats and 18 Republicans who signed the resolution have said publicly that they will switch only if the administration comes up with a compromise along the lines of what has become known as "the Glenn plan."

That is a reference to Sen. John Glenn (D-Ohio), who proposed the joint-command leasing idea during Haig's testimony Thursday.

Glenn argued that Saudi Arabia's sensitivity about its sovereignity might be overcome by convincing Saudi leaders that it is in their and the U.S. interest to have an AWACS intelligence capability in the Persian Gulf region and that a joint venture would permit the United States to provide a far more sophisticated system than is presently contemplated.

Some administration sources, noting the number of senators flocking to associate themselves with the idea, said Glenn's background as an astronaut and Marine officer gave his proposal a mantle of technological expertise.