Sixty-four senators have told President Reagan in a letter that they "vigorously" oppose the reported administration plan to sell $1.4 billion in arms to Saudi Arabia.

The senators told the president that such "an unwarranted Saudi arms request" would provoke "an unnecessary and unproductive confrontation" between Congress and the White House at a time when a number of controversial issues now before legislators require bipartisan cooperation.

"We feel constrained to oppose it vigorously," they wrote.

Sponsored by Sens. Alan Cranston (D-Calif.) and Bob Packwood (R-Ore.) and signed by 46 Democrats and 18 Republicans, the letter was sent Friday but not released to the news media until yesterday to avoid embarrassing Defense Secretary Caspar W. Weinberger, who was visiting Saudi Arabia over the weekend.

The large number of senators, three short of the 67 needed to override a presidential veto, indicates that the administration is likely to face a major battle over another arms sale to the Saudis. It comes at an extremely delicate time when Saudi cooperation has become crucial to U.S. military forces escorting reflagged Kuwaiti oil tankers through the Persian Gulf.

Although the White House and State Department have begun consultations with key House and Senate committee leaders, the administration has not yet notified Congress of the sale.

Congressional opponents expect the administration to start the informal notification process mid-week in order to force a decision on the sale before Congress goes home in mid-November.

The administration had hoped to quietly work out an understanding with Congress in advance concerning which items it will support as part of the package.

According to initial administration information, the sale would include 12 F15 C/D fighters worth $502 million; 1,600 Maverick antitank missiles, $360 million; electronic countermeasures equipment and advanced avionics for older Saudi F15 aircraft, $300 million; and M60 tank improvements and artillery, $242 million.

The administration has also been sounding out congressional leaders on providing Bahrain with 70 Stinger antiaircraft missiles and 14 launchers to defend its gulf oil platforms and installations from possible Iranian attack.

The most controversial items are the new F15 aircraft and the Maverick and Stinger missile sales. As a compromise, the administration has suggested a limit be set on the number of F15s and Mavericks stationed at any one time in Saudi Arabia, with the remainder kept here as replacements.

The Saudis have 57 F15s and about 1,500 Mavericks. A senior U.S. official recently suggested a ceiling of 60 F15s.

The complicated sale process involves a 20-day informal notification period, followed by a 30-day period during which Congress can pass a resolution of disapproval aborting the sale. The president can veto the resolution but Congress can override it by a two-thirds vote of the House and Senate.

In their letter, the senators said they oppose the plan because, "the Saudis have not made substantial efforts to achieve progress in the Middle East peace process and they continue to fund terrorist organizations like the PLO."

The letter makes no mention of the Saudi contribution to U.S. military forces now in the gulf. But in a Sept. 16 "Dear Colleague" letter, Cranston criticized the Saudis for refusing to supply these forces with "any meaningful landing rights and shore-based resupply capabilities."