Just as a transpacific telephone call asked Vice President Walter Mondale's help with Sen. Muriel Humphrey's vote for the Mideast arms deal, Sen. Frank Church was preparing a sudden pro-Israeli shift that stunned his Senate colleagues and threw the administration into disarray.

That rapid sequence of events, culminating in the 8-to-8 Senate Foreign Relations Committee vote May 11, casts a long and ominous shadow ahead for President Carter. It raises nagging questions about the dependability of Church, the 21-year Senate veteran from Idaho, who becomes committee chairman next January with the retirement of Sen. John Sparkman (D-Ala.)

It was future chairman Church who served notice that he would support the aircraft package with certain specified changes. It was future chairman Church who, at a private breakfast with Secretary of State Cyrus Vance and other key senators May 9 in Vance's dining room, gave all present to understand that changes offered by the president satisfied him.

Nothing had been left to chance. Vance, National Security Director Zbigniew Brzezinski, Mondale and four top-ranking senators - Majority Leader Robert Byrd, Republican leader Howard Baker, Sparkman and Abraham Ribicoff (D-Conn.) - had spent hours in private sessions with Church. Their mission: to reshape the president's aircraft deal to Church's specifications.

Mondale, just arrived in Honolulu May 10, was informed from Washington that fellow Minnesota Democrat Muriel Humphrey was wavering and promptly telephoned her. Mrs. Humphrey's vote was essential to give the Church-fashioned compromise a majority vote in the Senate Foregin Relations Committee. Interim Sen. Humphrey decided to stick with the president, despite pressure on her by the pro-Israel lobby, which one fellow senator called "brutal."

Church was under similar pressure from Jewish Americans who contributed heavily to his 1976 presidential campaign against Jimmy Carter. He wilted. Lamely explaining May 11 to shocked committee colleagues, Church took refuge in the late Sam Rayburn's explanation for changing his mind about Lyndon Johnson's running for vice president in 1960. "I'm a wiser man today than I was yesterday," Church said.

Church's colleagues were not impressed. With his vote, the committee would have given the president a clear 9-to-7 victory. That might have killed the issue once and for all in committee.

On the floor of the Senate shortly before the committee vote, a confident Byrd was counting the ayes and nays with Church. He was surprised when Church complained that he "could not get a consensus" for the compromise.

"But," he told Church, "I count nine votes for the president." Church said yes, but that would include his own vote and he was going to vote no. Byrd could scarcely believe his ears.

The long road toward a compromise acceptable to Church had started weeks earlier. Two days after Baker told CBS's "Face the Nation" April 16 that he could support F15 warplanes for Saudi Arabia with a little "tinkering," Church privately informed Senate leaders he, too, could go along if changes were made.

Church wanted the package split up; he asked assurance that the F15 jets for Saudi Arabia be disarmed for offensive purposes; and he demanded Saudi agreement not to base the planes near the Israeli border. With those changes, he remarked privately, he could "support these planes for Saudi Arabia in the national interest."

That word was delivered April 21 to Mondale and Brzezinski at the White House. Vance was then in Moscow, but approved Church's changes on April 27 at breakfast with Church, Baker and other senators. But at that meeting, Church raised a new question: Could the Saudi shipment [WORD ILLEGIBLE] planes be reduced? Impossible, said Vance [WORD ILLEGIBLE] extra F15s could be sold to Israel. Church appeared pleased.

On May 9, at yet another Vance breaking Church raised the bidding. He asked for assurances that the president never again would couple Israel with another country.

Without a flat commitment, Carter met Church halfway. In his formal letter to Congress asking approval of the sales, he included a written-for-Church "addendum" calling the three-nation sale unique. That suggested no repetition.

For one day following that breakfast, Church was an advocate of the arms deal. On May 11, complaining he lacked a "consensus," he flipped back to his old position. But other senators - Baker, Humphrey, Charles Percy (R-Ill.) and John Glenn (D-Ohio) - had been influenced by Church's persuasive arguments two days earlier, they stayed with the president.

That pattern by soon-to-be chairman Church was the sourest note in a bitterly fought battle that showed Jimmy Carter at his best. "With friends like Frank Church," said one Democratic senator, "the president needs no enemies."