Though many administration officials and members of the Senate can share the credit for President Carter's victory on the first Panama Canal treaty, one Republican made it possible; Howard H. Baker Jr. of Tennessee.

Had Baker decided to oppose the treaty - and he had countless good reasons to do so - there seems no real chance that it could have won two-thirds approval in the Senate. As the Senate's minority leader, Baker's support gave the proponents the banner of bipartisanship to dignify their cause. More immediately, Baker's own vote and those of the handful he certainly swayed would have provided the margin of defeat, had they gone the other way.

But Baker lined up with the president, the Senate establishment and the liberal-to-moderate Washington mainstream.

His allies in the fight for the treaties included a lot of people who regard Howard Baker suspiciously, seeing him as a little slippery and a lot ambitious. These people would not have been at all surprised to see Baker go the way of other would-be Republican candidates for president like Bob Dole (Kan.) and Richard S. Schweiker (Pa.) both of whom opposed the treaty, Dole vociferously.

Nor would Baker's Republican colleagues have been surprised. The minority leader has positioned himself right at the middle of the Republican spectrum; he is fully acceptable to both wings of the party - or he was until the Panama Canal treaties appeared.

Moreover, the Republican right had sought to make the vote on the canal treaties the Wassermann test of GOP presidential politics. More than one Republican conservative declared that no Republican who hoped to win the party's nomination for president could vote yes on the treaties.

Baker does have presidential ambitions, and he understands the power of the right within the Republican Party.

In an interview last week before the final 63-to-32 vote, Baker noted that there was also important Republicans who favored the treaty, particularly former President Ford, but some conservatives too, like William F. Buckley, the editor and columnist. "When there's a split," Baker said, "it becomes - as it should be - a matter of individual choice."

Baker first heard that the United States and Panama were about to agree to new draft treaties last August. He was home in Tennessee when President Carter called him on the telephone. Carter said agreement was imminent, and he hoped the Senate could dispose of the treaties quickly.

Baker said he told Carter at once that there would be strong opposition if the treaties came up for a vote during 1977, and that the country had to be "educated" before a vote. Soon afterward he and the Senate majority leader, Robert C. Byrd (D-W.Va.) - who was as important as Baker to Carter's ultimate victory - agreed that the Foreign Relations Committee ought to use the rest of 1977 to hold hearings on the treaties.

In early September, the two principal American treaty negotiators, Ellsworth Bunker and Sol Linowitz, visited Baker on Capitol Hill. Baker told them then he was inclined to support the treaties, but he had to study them.

Baker recalled that he had looked at the original, 1903 treaty some years ago when Sen. Strom Thurmond (R-S.C.) asked him to sign a resolution opposing any change in the canal's status. It struck him then, he said, that the treaty needed to be changed.

But Baker also understood the political cost that might be exacted if he supported the new treaties, and when he studied the new ones, he found them disquietingly vague.He decided, obviously, to proceed with great care.

By his own account, Baker phoned former Secretary of State William P. Rogers and other acquaintances to ask who might serve as consultants to him on the treaty issue. He wanted one expert who opposed the treaties, one who favored them. He ended up with Prof. Roger Fontaine of Georgetown University, an opponent, and William D. Rogers, not the former secretary but a former under secretary of state under Henry A. Kissinger and a Latin American expert who supported the treaties.

Fontaine and Rogers prepared memos, briefs and discussions for Baker and his staff. All the participants in this process say now it was exhaustive. "It appealed to me as a lawyer," Rogers said, "a good adversary proceeding."

Baker, meanwhile, shared the papers his consultants produced with other Republican senators. From the beginning he has gone to great lengths not to offend any other Republicans personally, a tactic that appears to have been successful.

In December Baker made his decision - to support the treaties, provided they were slightly altered by amendment. The senator and his two consultants all say that the decision was on the merits, not because of some political calculations.

Not that politics were ignored - Baker regularly discussed the political implications of the treaty fight with his inner circle of advisers.

Politics for Baker meant more than his future in the GOP. He is running for re-election in Tennessee this year, and the pressures from home were substantial. The American Conservative Union had taken out full-page newspaper ads in the state which said in big letters: "Sen. Baker alone can save the Panama Canal." At a University of Tennessee football game in early November, with 84,000 fans in the stadium, an airplane flew overhead throughout the game carrying a banner that said, "Let Sen. Baker Hear From You On The Canal." In the end Baker received 64,000 pieces of mail on Panama, almost all of it against the treaties.

(Though Baker and his aides talk a lot about these pressures, the primary opponent they feared has not materialized, and Baker's Senate seat is not in jeopardy).

No doubt at the suggestion of U.S. officials, Gen. Omar Torrijos, the Panamanian leader, had been urging Baker to visit Panama. At first baker declined, but in late December he decided to go. Several colleagues speculated at the time that his change of heart was produced by a political calculation that the intense, right-wing opposition to the treaties had peaked, and public opinion might be going the other way.

Baker went to Panama ready to declare his support for the treaties, provided they were amended, particularly by spelling out U.S. rights after the year 2000 the way Torrijos and Carter had spelled them out in an October communique. (That document had not been signed; Baker wanted it included in the treaties.)

But, ever careful, Baker couched this news in what sounded to many like negative terminology. In Panama, he told Torrijos then announced publicly that he could not support the treaties in the form they were negotiated. Most press accounts of the trip implied that Baker had come out against the treaties.

In fact, Baker had determined in conversation with Torrijos that the changes he sought would be acceptable to the Panamanian. Moreover, he knew that many senators would be positively influenced by the changes he demanded. Though Baker would anger some by his ultimate [WORD ILLEGIBLE] in support of the pacts, he was also in a position to take credit for important modifications in them that many other senators would eventually cite as the basis for their decisions to vote yes.

Baker's key changes ended up as "leadership amendments" sponsored by him and Sen. Byrd, and quickly cosponsored by most of the Senate. (In fact, Sen. Ernest F. Hollings had earlier proposed the same changes.) They were the only amendments adopted to the treaty the Senate approved Friday. (Several reservations were also approved.)

Baker says he is confident that his political future has not been irreparably damaged by his role in Thursday's vote. He has taken pains to avert such an eventuality. Last month, for example, he invited the leader of Republicans opposed to the treaties, Sen. Paul Laxalt (Nev.), to speak at a Lincoln Day dinner in Tennessee, and Laxalt said there he would continue to support Baker as minority leader, regardless of the treaty dispute.

In an interview, Baker said, "not one single, credible soul" has accused him for forfeiting his political future with this decision.

In fact, some sympathetic observers have speculated that Baker's contribution to Carter's victory may have enchanced his future prospects - though this is admittedly a speculative assessment.

According to it, Baker could have gained nothing by simply accepting the Diktat of the far right and voting against the treaty. "A Republican nomination for president won at the price of accepting the demands of the right wing isn't worth much," one Senate source put it.

But now, Baker has cast himself as the sort of statesmanlike, moderate and pragmatic Republican who might have a chance to defeat a Democrat were he selected to run for president, this analysis continues. If a Republican convention will have him, he might have a chance to win.

That line of reasoning occurred to more than one of the Carter administration officials who found themselves allied with Baker in the lobbying effort on behalf of the treaties.

"I got to really like the guy," one administration man admitted. Then he paused. "The sonofabitch scares me, if you want to know the truth."