President Reagan yesterday continued his campaign to coax a majority of the Senate into supporting his proposed $8.5 billion sale of sophisticated radar surveillance planes and other aircraft equipment to Saudi Arabia.

The Senate will vote the week after next on a resolution to disapprove the sale of the Airborne Warning and Control System planes, and Reagan talked with eight senators at the White House yesterday in an effort to win their backing.

Whether the administration will gain a majority remains unclear, but the White House insisted that the gap is narrowing and that it believes the battle can be won.

"We're higher up the mountain than we were before," White House communications director David Gergen said. "We're still behind, but we're definitely closer."

Of the senators who talked with Reagan, J. Bennett Johnston (D-La.), who had been leaning in favor of the sale, formally announced his support. Sen. Patrick J. Leahy (D-Vt.), who has been regarded as leaning against, left the White House saying he is undecided.

Predictions by aides to Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) that Sen. Mark Andrews (R-N.D.), a co-sponsor of the disapproval resolution, would switch sides failed to materialize. Aides to Andrews said he is considering the president's arguments but remains opposed.

Some congressional sources said many senators being wooed by Reagan have expressed interest in his attempt to gain a greater share of U.S. control over the AWACS planes when he attends an economic summit meeting in Cancun, Mexico, next week.

Gergen said there is a "good chance" that Reagan will confer with Crown Prince Fahd, the effective head of the Saudi government, in Cancun. Gergen said he does not expect the meeting to involve discussion of possible new Saudi concessions on the AWACS control issue.