Fifty Senators--half the Senate's membership--yesterday joined as cosponsors of a resolution to block President Reagan's plan to sell sophisticated radar planes and other aircraft equipment to Saudi Arabia. Their announcement dealt the $8.5 billion deal a serious blow.

In taking their action, the 32 Democrats and 18 Republicans led by Bob Packwood (R-Ore.) spurned a strong plea by Secretary of State Alexander M. Haig Jr., who told the Foreign Relations Committee that approval of the sale is essential to U.S. strategic goals in the Mideast as well as the Reagan administration's credibility in the conduct of foreign policy.

When the resolution first was unveiled by several of the signers at a news conference, they said there were 51 cosponsors. Later, however, one of those listed, S. I. Hayakawa (R-Calif.), said that while he is leaning against the sale, he had not yet made up his mind and that his name had been included as the result of a "misunderstanding" between him and Packwood.

The resolution was taken on Capitol Hill as proof the administration faces a steep uphill battle to overcome congressional opposition to the sale and might be able to avoid a humiliating defeat only if it works out a new arrangement with the Saudis that would keep the Airborne Warning and Control System (AWACS) planes under U.S. control or eliminate them from the sales package.

Saudi Arabia so far has rejected these options as unacceptable. In testimony yesterday before the Foreign Relations Committee, Haig, while not explicitly ruling out attempts to renegotiate the package, implied that the Saudi stance is unyielding when he said that options such as joint U.S. control of the AWACS is "just not a practical solution."

For the sale to be blocked, both houses of Congress must vote against it. Administration sources privately concede they have almost no hope of reversing an apparent strong majority against the sale in the Democratic House, and they have concentrated their lobbying on the presumably more sympathetic Republican Senate.

David Gergen, White House communications director, said late yesterday that the administration intends to continue that approach. He told reporters: "The president does not believe this is the end of the struggle. He thinks the struggle is just beginning."

Gergen added that White House lobbyists have indications that some of those who cosponsored the resolution, initiated by Packwood, are still open-minded and can be dissuaded from their present intention to vote against the sale.

Haig, noting that the main objection to the sale is rooted in concern that the AWACS planes will pose a danger to Israel's security, said the administration has worked out four secret agreements with the Saudis to prevent that from happening.

In broad outline, he said, they involve limitations on transfer of the equipment and intelligence information collected by the AWACS to third parties, access by the United States to the collected information, provisions for the physical security of the equipment and technology and limitations on the geographic areas where the Saudis would operate the planes.

Haig said the administration has assembled three teams, representing the White House, the State Department and the Defense Department, to brief members of Congress privately about the details of these agreements. It is the administration's belief, he asserted, that once the members have the necessary information, their concerns about Israel's security will be allayed.

Packwood and other senators who worked with him on the resolution conceded that this could happen between now and late October, when the Senate is expected to vote on the resolution. But the dominant opinion in Senate circles last night seemed to be that Reagan cannot win the fight without a revision of the sales package far more radical than his administration and the Saudis currently are willing to make.

The 50 signatures lined up for the resolution by Packwood, in collaboration with Democrats Alan Cranston (Calif.) and Henry M. Jackson (Wash.), are still a vote short of the majority needed to block the sale in the Senate.

Packwood said yesterday that two other Republicans have assured him privately that they intend to vote against the sale, and Jackson added that he had similar assurances from four Democrats.

At issue is a controversy that began in 1978 when Congress declined to block then-President Carter's decision to sell 62 F15 jet fighters to Saudi Arabia. That sale went through largely because of assurances to Congress that the jets would not have an offensive capability permitting their use against Israel.

The deal has been opposed strongly by Israel, and during an official visit here last week, Israeli Prime Minister Menachem Begin, despite efforts by administration officials to dissuade him, reiterated that his country's objections remain in force despite Reagan's promise of closer U.S.-Israeli military cooperation.

The 50 Senate Signatories To Packwood's Resolution

Following is the list of the 50 senators

REPUBLICANS

Andrews, North Dakota; Boschwitz, Minnesota; Cohen, Maine; D'Amato, New York; Danforth, Missouri; Durenberger, Minnesota; Gorton, Washington; Hatch, Utah; Hawkins, Florida; Heinz, Pennsylvania; Jepsen, Iowa; Kasten, Wisconsin; Packwood, Oregon; Pressler, South Dakota; Roth, Delaware; Simpson, Wyoming; Specter, Pennsylvania, and Weicker, Connecticut.

DEMOCRATS

Baucus, Montana; Bentsen, Texas; Biden, Delaware; Bradley, New Jersey; Burdick, North Dakota; Cannon, Nevada; Chiles, Florida; Cranston, California; DeConcini, Arizona; Dixon, Illinois; Dodd, Connecticut; Eagleton, Missouri; Ford, Kentucky; Hart, Colorado; Heflin, Alabama; Inouye, Hawaii; Jackson, Washington; Kennedy, Massachusetts; Levin, Michigan; Matsunaga, Hawaii; Mitchell, Maine; Metzenbaum, Ohio; Moynihan, New York, Pell, Rhode Island; Proxmire, Wisconsin; Pryor, Arkansas; Riegle, Michigan; Sarbanes, Maryland; Sasser, Tennessee; Tsongas, Massachusetts; Williams, New Jersey, and Zorinsky, Nebraska.