President Reagan called six senators to the White House yesterday as part of his campaign to prevent a congressional veto of his $8.5 million aircraft sale to Saudi Arabia, but the effort failed to turn up any new converts to the president's cause.

Three senators--Alan J. Dixon (D-Ill.), Daniel K. Inouye (D-Hawaii) and Roger W. Jepsen (R-Iowa)--left the White House declaring that they were unconvinced by the president's arguments and intend to vote against the sale of Airborne Warning and Control System (AWACS) planes when the Senate considers it next Wednesday.

Sen. John Melcher (D-Mont.) said he still is undecided and will not make up his mind until after the weekend. Sens. Malcolm Wallop (R-Wyo.) and Jennings Randolph (D-W.Va.), both declared supporters of the sale, reaffirmed their backing.

The net effect, according to Capitol Hill sources, was to leave 52 senators opposed or leaning against the deal, approximately 40 in favor and eight undecided. Reagan's intense lobbying campaign is aimed at shifting that lineup sufficiently to prevent the Senate from following the lead of the House and adopting a resolution of disapproval that would block the sale.

White House communications director David Gergen, saying Reagan remains "cautiously optimistic," disputed the contention that the opponents still can count on a majority. He said, "We believe that they're short a majority--that there's a pool of uncommitteds and that it's from this pool that it will be decided."

Gergen, noting that the Senate tentatively is expected to vote at 5 p.m. next Wednesday, said Reagan doesn't expect to do any long-distance lobbying while he is in Cancun, Mexico, attending an economic summit the remainder of this week. But, he added, the president expects to keep up his campaign to the last moment and will talk with additional senators next Monday, Tuesday and Wednesday.

This Friday, Reagan will meet for an hour with Saudi Crown Prince Fahd in Cancun. However, both Gergen and Secretary of State Alexander M. Haig Jr., at a briefing on the Cancun summit, warned against expectations that the meeting might produce some new agreement over control of the AWACS planes that would make the deal more palatable to the Senate.

"Don't expect any bombshells on AWACS," was the way Gergen put it.

Senate Minority Leader Robert C. Byrd (D-W.Va.), whose position could influence some uncommitted senators, said yesterday he will reveal shortly how he intends to vote. Byrd was careful not to say what he will do; and, in an apparent reference to the fact that most Senate Democrats are opposed to the sale, he asserted that he does not consider it a partisan issue.

However, he strongly criticized the administration's handling of the sale, which angered many senators because of alleged White House failures to consult them and take their concerns into account. Byrd said, "I've never seen anything more badly botched than this one."

As part of its lobbying campaign, the White House plans to send the Senate a letter in which Reagan will give written assurances about the control and safeguarding of the AWACS and other equipment in the sale. The letter has been held up, apparently in hopes that it will serve as a device to allay the concerns of wavering senators. Gergen refused to say yesterday when it will go to Capitol Hill.

"It's always been our plan that it would go up late in the game," he said. "But I can give you no precise hour or day."