Through its fog of unpublished understandings and reassuring rhetoric, President Reagan's strategy in the selling of the AWACS emerged most clearly in the weeklong care and stroking of Sen. Larry Pressler.

The conversion of this South Dakota Republican, who was once signed, sealed and apparently delivered as an opponent of the sale of the sophisticated radar planes to Saudi Arabia, was accomplished without grand policy shifts or compromises.

Instead, in this effort that the White House views as a battle between America's president and America's Jewish lobby, Ronald Reagan successfully converted Larry Pressler to his cause with a bit of politically orthodox, rabbinically detailed attention to the little things that apparently mean so much.

The machinations behind Pressler's conversion are worth studying, because they apparently hold the key to whether the president will be able to gain safe passage for the Airborne Warning and Control System through the turbulent airspace of the Senate, en route to Saudi Arabia.

Opponents of the sale still hold a slim majority, counters for both sides say.

"I'm realistic enough to know that when I'm lobbied someone wants something," Pressler explained, struggling to put it all in perspective after switching sides just moments before the Senate Foreign Relations Committee voted on the sale.

The South Dakotan, who is best known for having brought to the Senate his handsome countenance and youthful enthusiasm, had just undergone a heady, hard-sell lobbying campaign that had him spending almost as much time with the president last week as did Nancy Reagan or Edwin Meese III.

Which just shows what a difference a week makes. Seven days earlier, during his committee's hearings on the sale, Pressler had been peeved at all the president's men. And the feeling was apparently mutual.

"I've been having trouble with the White House," he had confided glumly then. "They won't talk to me. They won't even return my calls."

One of his calls, Pressler said, was merely an attempt to find out when the president would be signing a minor bill of his that had been passed by the Senate and House.

Pressler reasoned that he had incurred presidential pique because he had signed on as a cosponsor of the anti-AWACS resolution and he had voted against the nomination of Ernest Lefever, Reagan's choice to handle human rights.

So, at that AWACS hearing, when Pressler found himself temporarily sitting as chairman while Secretary of State Alexander M. Haig Jr. was testifying, the South Dakotan told Haig's assistant secretary for congressional liaison how he wanted to attend White House ceremonies honoring Raoul Wallenberg, who had saved thousands of Hungarian Jews during World War II. Richard Fairbanks said he'd handle it.

But minutes later, a clearly embarrassed Fairbanks delivered to Pressler a message handwritten on a "Secretary of State" note pad:

"Dear Senator,

"I checked with the White House congressional office re: the signing ceremony. I am sorry to have to tell you it is already full. Dick Fairbanks."

Back at the White House, a high-level aide termed Pressler's request "silly." He elaborated, "Most senators don't invite themselves to White House ceremonies."

What happens, it turns out, is that senators invite other senators to the White House.

Pressler's road to the White House was paved by Senate Majority Leader Howard H. Baker Jr., who a week ago included the South Dakotan in a group of five that he took to the Oval Office for some presidential persuasion.

"There were five of us in the Oval Office, but most of the attention was on me," Pressler recalled.

Last Tuesday, Baker brought Pressler back for another round of Reaganization. This time it was one on one.

The president talked about how important it was for foreign nations to know that the president's commitment in international negotiations is America's commitment, Pressler said. "He said, 'I just won't be able to conduct foreign policy if I lose this,' " Pressler recalled. "He was almost emotional on it."

Pressler is asked whether he believes Reagan was equally concerned about this importance of a presidential commitment in foreign affairs back when he fought then-President Carter's treaty commitments on the Panama Canal and SALT II. "Well, that's a good point," the senator said. "I guess every president makes that argument."

Pressler was, by this time, trying to find a publicly graceful way of getting himself out of the signed-in-opposition column and over to the president's side.

He came up with a resolution of his own that would have the United States provide Israel with radar jamming equipment, the reasoning being that Israel could render the AWACS useless if the Saudis ever used it to monitor Israeli military actions.

But the resolution apparently will provide Pressler with more cover than it does the Israeli military. "The problem is not jamming the AWACS," said an Israeli official. "It doesn't matter if we are provided with this equipment or not."

Midafternoon Thursday, as Pressler was sitting in his committee hearing room just before the vote, there came the final calling.

The president, who was out of town making a speech, was telephoning for one last appeal.

Pressler said Reagan told him, "I know your concerns. You've said them to me twice, and we're going to meet your concerns."

The president never said how he would meet Pressler's concerns, but Pressler says he feels that the president has "somehow" assurred him that the Saudis will join the Mideast peace process and that the Saudi monarchy will remain stable.

From what the president told him, he said, "the deal has changed substantially . . . our copilots will be there aboard Saudi-controlled AWACS until 1995, possibly." But White House aides say it has not changed at all. The arrangements about Americans in the cockpit into the 1990s is not a concession newly won -- it was testified to by Haig in front of Pressler's committee.

Pressler reflected on it all the morning after his conversion:

"I must admit, sitting in the Oval Office and having the president of the United States making changes that he says are changes -- it's very persuasive. And I'm not a new guy. But I must admit, it's a very impressive thing to have happen to you."

He paused, then added:

"It just amazes me that for a man his age he is that alert. I have a father in a nursing home who is younger than he is--so I know."

EPILOGUE: Back at the White House, on the morning after, reporters were handed a series of routine news releases. One said simply that the president had signed a bill, SB 308, which dealt with tourism. A White House aide was asked who authored the bill. He replied:

"It is Mr. Pressler's bill."