President Reagan summoned a large group of senators to the White House yesterday to kick off what the administration and congressional leaders think is an uphill fight to keep Congress from blocking an $8.5 billion sale of radar spy planes and sophisticated aircraft equipment to Saudi Arabia.

Twenty-seven senators were invited to hear the president's arguments in favor of the sale, and several of those who attended or who talked with the president separately said afterward Reagan is aware that a majority of the Senate is inclined to vote against it.

Typical was the comment of Sen. Paul Laxalt (R-Nev.), a staunch Reagan loyalist, who said: "If the vote were conducted today, I'd say we don't have the votes to sustain the sale. It's going to be a very tough fight."

Laxalt's assessment appeared to be in accord with the statement Sunday by Sen. Alan Cranston (D-Calif.), one of those leading the opposition on the grounds that selling the equipment to Saudi Arabia would pose a danger to the security of Israel. Cranston asserted there now is a Senate majority of 51 votes in favor of vetoing the deal.

But Cranston also conceded that the president, if he lobbies intensively in weeks ahead, could persuade some senators to change their minds. That point also was made yesterday by Laxalt, who said, "I rather think that by the time the president indicates to our colleagues in the Senate the basic reasons why we should make the sale, we'll eventually be able to win."

At issue is the administration's intention to sell the Saudis five Airborne Warning and Control System (AWACS) radar surveillance planes, plus enhancements for F15 jet fighters already promised to the Saudis and other equipment.

Although Saudi Arabia is America's principal supplier of oil, it also is a sworn enemy of Israel. And Prime Minister Menachem Begin, on a visit here last week, reaffirmed the Israeli view that the proposed sale poses a great danger to his country.

The sale will be scuttled if majorities of both houses vote against it within 30 days after the administration formally notifies Congress of its intent. That notification is expected to be made at the end of the month.

It is believed there is a strong majority against the sale in the House, and the administration is concentrating on thwarting opponents on the Senate, where the vote is expected to be much closer.

Several senators at yesterday's meeting said they told Reagan they would give the administration a chance to make its case.

But, they added, while the president promised more detailed briefings and testimony in days ahead, he confined himself to making only general arguments about why the sale is in the U.S. national interest.

In a related development yesterday, the State Department agreed with Begin's contention that there is no direct link between the outcome of the AWACS vote and the agreement reached last week for broadened security cooperation between the United States and Israel.

Yesterday, State Department spokesman Alan Romberg, in a carefully phrased response to questions, said the administration is seeking a comprehensive security strategy to combat Soviet influence in the Middle East and added that there is "no strict conditionality" attached to the agreement for greater military cooperation with Israel.

Secretary of State Alexander M. Haig Jr. suggested to reporters on his flight here from West Germany yesterday that U.S. policy toward Israel would be altered if Congress prohibits the AWACS sale to Saudi Arabia, the Associated Press reported.