As Congress began a week-long recess yesterday, the intensified lobbying effort mounted by President Reagan in the wake of Anwar Sadat's assassination has induced nine senators to declare their backing for the controversial sale of Airborne Warning and Control System (AWACS) planes to Saudi Arabia.

Two--Orrin G. Hatch (R-Utah) and Alan K. Simpson (R-Wyo.)--are defectors from the group of 50 senators who earlier had announced their intention to vote against the deal. The other seven, including Nancy L. Kassebaum (R-Kan.) and Don Nickles (R-Okla.), who declared their support yesterday, all had been expected to vote for the sale.

Despite the hard-sell tactics of White House lobbyists, congressional sources seem to agree that, as of now, at least 55 senators are firmly opposed or leaning strongly against the sale, while the administration's support numbers between 30 and 40.

The Saudi deal--the largest arms sale in history--will be blocked if both houses of Congress vote against it before the end of the month. The administration concedes it has no chance of prevailing in the Democratic House, and it has staked its hopes on turning the numbers around in the presumably more sympathetic Republican Senate.

At present, the main thrust of administration strategy is to cite the death of Sadat--America's closest friend in the Arab world--as evidence of the need to cultivate the cooperation of other moderate Mideast regimes like Saudi Arabia's and to appeal to Republican senators to show their loyalty to Reagan as he confronts his first major foreign policy test.

Opponents of the sale point out that, except for Hatch and Simpson, the combination of pressures and pleas by the administration has shown no results. Instead, these opponents note, the White House has been forced to try to wring the maximum publicity out of the public declarations of support from senators whose sympathy for the administration position never was in doubt.

Administration strategists reply that their new campaign, launched Wednesday when Reagan met with 43 GOP senators at the White House, is not meant to produce any dramatic, overnight swing toward the president. These sources describe their efforts as a gradual, chipping-away process that they hope will build up a pro-administration majority by Oct. 20 when the sale is tentatively expected to face a vote by the full Senate.

As a companion move to its calls for party loyalty, the administration continues to cast about for a compromise that would satisfy the concerns of many senators about the alleged lack of safeguards against misuse of the AWACS planes. These concerns involve fears that the equipment might be used against Israel or fall into the hands of U.S. foes like the Soviet Union.

One proposed compromise--a resolution originated by Sens. Sam Nunn (D-Ga.) and John W. Warner (R-Va.)--would call on the president to certify in writing the safeguard "understandings" reached with the Saudis. Reagan has agreed to do that, but the non-binding nature of the resolution is regarded as insufficient.

Late Wednesday, top administration officials began discussing with a group of freshman Republican senators a variation of the Nunn-Warner plan that would require Reagan's certification of the safeguards before any AWACS planes are transferred.

Congressional sources have cautioned that it is too early to tell whether this initiative will prove promising, but it has aroused considerable interest because of the administration's attempt to enlist two declared opponents--Slade Gorton (R-Wash.) and Robert W. Kasten Jr. (R-Wis.)--in drafting and supporting this latest compromise plan. Neither senator has so far indicated a willingness to switch sides.