President Reagan pleaded with Republican senators yesterday to support his sale of radar planes to Saudi Arabia but, in the first congressional test of the deal, the House Foreign Affairs Committee voted overwhelmingly against him.

"I need you, your country and the world needs you on this decision," Reagan told 43 GOP senators as he asserted that the assassination Tuesday of Egyptian President Anwar Sadat makes the sale even more vital to the success of U.S. Mideast policy.

It was not clear whether Reagan's appeal would produce the necessary number of converts, although Sen. Alan K. Simpson (R-Wyo.), previously opposed to the Saudi deal, announced that he is switching to Reagan's side.

While Reagan was making his hard sell, the Democratic-controlled House committee turned aside pleas by Republican members for a delay and voted, 28 to 8, for a resolution disapproving the $8.5 billion aircraft sale.

Only three Democrats and five of the committee's 16 Republicans backed Reagan. Among those voting against was the committee's ranking Republican, Rep. William S. Broomfield (Mich.), although, in an unusual reversal of roles, committee Chairman Clement J. Zablocki (D-Wis.) supported the administration.

The net effect of yesterday's flurry was that no one knows how the Sadat assassination will affect the emotional debate about whether the United States should sell the Airborne Warning and Control System (AWACS) planes and other sophisticated aircraft equipment to the Saudis.

By late yesterday, at least two efforts were under way by different groups of senators, with covert administration backing, to hammer out resolutions that a majority might see as acceptable compromises enabling them to vote for the sale.

Although administration sources pointed to these stirrings as signs that the momentum is starting to move toward the president's side, more neutral sources within Congress cautioned that it still is too early to tell whether these compromise efforts will get anywhere.

The deal will be blocked if both houses of Congress vote resolutions of disapproval by the end of the month. Prior to Tuesday's traumatic events in Egypt, it appeared that the House was solidly against the sale and that the administration faced an uphill battle in the Senate.

On its face, the murder of Sadat would seem to strengthen the opposition argument that the real danger to Mideast Moslem regimes such as that of Saudi Arabia comes from internal instability and that it is too risky to give them advanced U.S. weaponry that could fall into the hands of anti-American forces.

That point was made repeatedly by administration critics during the 2 1/2 hours of debate that preceded yesterday's committee vote.

However, the administration, spurning pleas to put the AWACS debate in abeyance for the time being, has seized on Sadat's death to argue to Republican senators, many of whom have strong doubts about the sale, that party loyalty dictates support of Reagan as he confronts his first major foreign policy test.

According to administration sources, this strategy does not envision any dramatic, overnight reversal of the Senate majority that had been leaning against the sale. Instead, the sources said, the aim is to bring about a process that, over the next week or two, will change Senate opposition to support for the president.

Simpson was the second Republican among 50 senators who cosponsored a resolution of disapproval to change his mind within the last 24 hours. On Tuesday, Sen. Orrin G. Hatch (R-Utah), citing changed circumstances brought about by Sadat's murder, announced that he intends to back the sale.

However, congressional sources said yesterday that Hatch has made clear to the White House and to Republican colleagues that his support depends on the administration fulfilling certain promises that he says Reagan personally gave him.

Specifically, the sources added, Hatch wants assurances there will be some form of written agreement under which the Saudis agree to operate AWACS planes only at a safe distance from Israel. They said he also wants to be assured that Israel can defend itself against any threat the planes might pose to its security.

A senior State Department official denied yesterday that Hatch had been promised any assurances beyond those that Secretary of State Alexander M. Haig Jr. described to Congress Monday as maximum concessions the Saudis are willing to make.

The congressional sources said, though, that Hatch insists he has been promised even stronger guarantees by Reagan and has put the White House on notice that if they are not forthcoming he again will oppose the sale.

Five other senators, all of whom had been leaning toward approval, yesterday announced they will back the arms deal. They are Republicans Gordon J. Humphrey (N.H.), Jeremiah Denton (Ala.), Richard G. Lugar (Ind.), John W. Warner (Va.) and Harry F. Byrd Jr. (Ind.-Va.).

As of last night, the principal effort to find a Senate compromise centered on a resolution introduced by Warner and Sen. Sam Nunn (D-Ga.). In broad outline, it states that AWACS technology should not be transferred to any country except under conditions roughly corresponding to the safeguard terms agreed to by the Saudis, and it would require the president to certify that these conditions have been met.

However, congressional sources said the nonbinding nature of the resolution appeared unlikely to satisfy those demanding some kind of joint control over the equipment.

In an attempt to explore a strengthened variation of the Nunn-Warner resolution, administration lobbyists, including White House chief of staff James A. Baker III, met late yesterday with five freshman Republican senators whose votes could be of key importance.

Under discussion, according to reliable sources, was a resolution that would require the president to certify in writing that U.S. interests would be protected and the safeguard issues resolved before any AWACS planes are transferred to Saudi Arabia. Since the first AWACS deliveries are scheduled for late 1985, that would provide a four-year grace period to work on the problem.

Those senators participating in discussion of this idea were Dan Quayle (Ind.), Mack Mattingly (Ga.), Frank H. Murkowski (Alaska), Slade Gorton (Wash.) and Robert W. Kasten Jr. (Wis.). Gorton and Kasten are among the 50 cosponsors of the Senate resolution to disapprove the sale.

Congressional sources said discussion among the five was still in a tentative state and added that more time will be required to see whether their potential initiative goes anywhere.

Meanwhile, these sources noted, the administration earlier had touted the Nunn-Warner resolution as an initiative that could turn the tide, and the sources cautioned that, for now, claims by both sides about whether the sale will be passed or vetoed should be regarded as psychological warfare by the contending lobbyists.

The sources pointed out that, as of last night, the administration's new, all-out lobbying drive had pried only two senators from the list of 50 cosponsoring the disapproval resolution.

Since several others not on the list are known to be leaning against the sale, the administration lacks the majority needed in the Senate and must maintain pressure on Republicans and efforts to find a viable compromise if its strategy of attrition is to work.