Senate Majority Leader Robert C. Byrd (D-W. Va.) has asked President Carter to withdraw at least temporarily his offer to sell seven highly sophisticated flying radar systems to Iran for $1.2 billion.

In a letter written to Carter Friday, Byrd said that because of the amount of work the Senate must complete before its statutory August recess, "it will be impossible for the Senate to give the proposal the careful and serious consideration it deserves."

The majority leader also expressed strong personal reservations about the wisdom of the proposed sale.

Byrd suggested that Carter resubmit the proposal to sell the Airborne Warning and Control Systems (AWACS), but not until next year. "I don't believe the sale is so critical it has to be finalized this year," he said, citing the projected 1981 date for first delivery to Iran.

The opposition of Byrd, who is probably the single most powerful force in the Senate, places the sale in serious jeopardy.

Byrd's letter was written the same day the Carter administration made its strongest public effort to date to persuade Congress not to block the proposed sale.

"Because the AWACS are central to Iran's long-range planning in the air defense field, which is vital to its national security, and given the absence of equally satisfactory alternatives, denial of the sale would be a very serious step," said Assistant Secretary of State Alfred L. Atherton in a prepared statement to the Senate Foreign Relations Subcommittee on Foreign Assistance.

"It would raise serious doubts in Iranian minds about the seriousness of our concern for Iran's security and about the future of our attachment to the long-standing U.S.-Iranian relationship," Atherton said.

At a morning press conference yesterday, Byrd said an impending filibuster over an election-financing bill, and Senate deliberations on a coal conversion bill that is a major part of Carter's energy package, "point to the need to delay action" on AWACS.

Motions have been filed in both the House and the Senate to block the AWACS sale. Both must be approved by Aug. 5 if Congress is to make its first use of a 1973 law that enables it to disapprove foreign military sales. The recess begins Aug. 7, Byrd said.

While Byrd's letter dealt largely with the parliamentary crunch facing the Senate, the majority leader said he also has reservations about selling a radar system so advanced that it has not been fully deployed by this country.

"It is unnecessarily risky to provide this highly sophisticated weapons system to a country that is on the border of the Soviet Union," Byrd said.

"I fear that by accident or defection or intelligence activities or diversion of classified technical manuals this information could fall into the hands of the Soviet Union."

Byrd said AWACS, with its ability to look down - a recent innovation in radar technology - at all activity in the 200-nautical-mile radius of surrounding airspace, could enable the Soviet Union to counter the low-flying cruise missiles that are an essential feature of this nation's nuclear arsenal.

The Soviets now have only a primitive look-down radar system, administration sources say, and are about 10 years away from developing an airborne radar system as effective as AWACS.

The White House had no immediate comment on Byrd's letter. Byrd said, he discussed the letter Friday and yesterday both with Secretary of State Cyrus Vance and with Carter, who "indicated recognition of the parliamentary problem which confronts us."

The arguments Byrd marshals in opposition to the AWACS sale are precisely the arguments the administration has targeted in its counteroffensive against the substantial opposition that has risen in Congress.

The opposition's strongest weapon thus far has been a General Accounting Office report that concluded the sale is not justified on security grounds. Included in the congressional watchdog agency's report is a letter from the Central Intelligence Agency director, Stansfield Turner, contending that a compromise of AWACS technology would be a serious loss for the United States.

At the Senate subcommittee hearing Friday, administration officials attacked the GAO report effectively enough to prompt Chairman Hubert Humphrey (D-Minn.) to announce he will call a hearing this week at which GAO and administration officials can confront each other and argue out their differences.

Administration sources say first that Iran has no air defense system, and that a ground-based radar system cited in the GAO report would be far more expensive and technically difficult than AWACS.

Next, they say the security risk is nowhere near as great as contended. The most sensitive communications equipment cited by opponents would not be on the AWACS sent to Iran, they say, and the computers on the plane are 1950s and 1960s technology - no mystery to the Soviets.

To arguments that Iran cannot, absorb the technology of the airplane - basically a modified Boeing 707 with a housing built by Westinghouse for the radar - they respond that Iran has technicians capable of handling 707s, which have been in Iran's arsenal for several years.

As for the potential for danger to the cruise missile, they say there is "serious question" whether an AWACS can detect a cruise missile, especially if the missile carries electronic counter-measure equipment.

Even if a whole AWACS should fall say administration sources, it is extremely unlikely that the Soviets could "back-engineer" enough to develop the capability to manufacture their own look-down radar. "They'd have one (AWACS), that's all," said one source.

Anyway, the sources argue. Iran has already received such sensitive equipment as the F-14 jet fighter equipped with the Phoenix missile, and has yet to lose any of it. The extremely tight security of the Shah's regime, they reason, may look like recession from one viewpoint, but from another it guarantees the safety of American weapons.