President Reagan yesterday vetoed a congressional resolution blocking a scaled-down arms sale to Saudi Arabia, but after hours of uncertainty in the Senate, the climactic test on sustaining the veto was put off until next month.

A long delay in lining up enough votes to uphold Reagan's action, followed by a Democratic threat to filibuster, combined to force the postponement. Senate leaders later agreed to vote on the veto on June 5, three days after Congress returns from a Memorial Day recess.

Reagan's veto message said that failure to provide arms to the Saudis "would send the worst possible message as to America's dependability and courage." He said the congressional action, if unchanged, would "damage our vital strategic, political and economic interests in the Middle East."

He said the Saudis had worked behind the scenes "to combat terrorism, which is as much, if not more, of a threat to them as it is to us." He also praised the Saudis for refusing requests for aid from Libyan leader Muammar Qaddafi. However, a White House official acknowledged that Saudi condemnation of the April 15 U.S. raid on Libya had been an obstacle to lining up Senate votes to sustain the veto.

Reagan accompanied the veto message with a letter to Senate Majority Leader Robert J. Dole (R-Kan.) in which he reiterated an agreement made earlier this week not to include 200 shoulder-fired Stinger antiaircraft missiles in the weapons package.

By late yesterday afternoon, Dole claimed he had enough votes to sustain the veto and allow the sale to go through. Democratic opponents of the sale conceded that Dole was right, but objected that some senators who oppose the sale had already left Washington. They threatened to delay final action on the issue past the Senate's scheduled adjournment time last night.

"What he [Dole] is faced with is the reality that there will be no vote today," Minority Leader Robert C. Byrd (D-W.Va.) said on the Senate floor as Dole listened. "There are some of us who are prepared to talk about this as long as we have to talk about it."

"I believe we have the votes to sustain the veto," Dole said, but he conceded that "as a practical matter we're not going to vote."

Sources said after the veto that personal pressure from Reagan, who talked to six senators yesterday, and efforts by Dole had produced 33 votes -- enough to sustain the veto with three Republican opponents of the sale absent. White House officials declined to predict whether they could muster the 34 votes that would be necessary to deny an override if all 100 senators were present and voting.

White House spokesman Larry Speakes said Reagan was "terribly disappointed" at the postponement but said the delay would not do "irreparable damage" if the Senate sustained the veto after the recess. Speakes also praised former president Jimmy Carter as "a wise man and patriot of the first order" for making telephone calls in behalf of Reagan's veto.

The Senate voted 73 to 22 on May 6 to block the sale, and the House, by an even more lopsided margin, followed suit the next day. On Tuesday, seeking to salvage the sale and avoid an embarrassing defeat in the Republican-controlled Senate, Reagan scaled down the size of the proposed arms package by removing its most controversial element -- the Stinger missiles.

Administration officials hoped that this move, cutting the proposed sale from $354 million to about $250 million, would allow them to obtain the votes necessary to sustain the president's veto of the disapproval resolution.

But despite a heavy White House lobbying campaign, it was not until midafternoon yesterday that Reagan felt confident enough to sign the veto message.

This was followed by further delays which, according to one Republican source, stemmed from uncertainty over whether Dole was correct in advising the White House he had the necessary votes. By the time Dole apparently had the votes, the Democrats, seeing their chances of blocking the sale dwindling, decided to caucus. They subsequently informed Dole that they would not allow a vote on the veto to be held yesterday.

The long day of waiting was not without comic relief. Shortly after Reagan vetoed the resolution, Ken Saunders, 30, a White House messenger, arrived in the Senate chamber with the veto message.

For about three hours, Saunders stood against the back wall of the chamber, clutching the president's message, while the few senators and staff aides on the floor studiously ignored him. Once Saunders was recognized, the veto would have been the Senate's pending business, and Senate GOP leaders were no longer certain that that was what they wanted.

Outside the chamber, while Saunders waited, Dole fell back on his sense of humor to make the best of what was clearly an uncomfortable situation. He said the Senate would act on the veto "as soon as we find the messenger."

"He's out walking around the Capitol," Dole said. "He's watering his horse."

Saunders, and the veto message, were not recognized until 6:45 p.m., by which time Senate leaders had agreed to postpone the issue.