April 6 was perhaps the darkest day in the Carter administration's long fight to win Senate approval of the Panama Canal treaties. That day the administration learned that Gen. Omar Torrijos, Panama's "maximum chief," had written letters to certain world leaders and to the United Nations suggesting that the "DeConcini reservation" to the Panama Canal neutrality treaty might provoke Panama to reject the treaty as approved in the Senate.

This was grave news. Panama had not told the United States anything about these letters, and they clearly violated Torrijos's earlier pledge to say or do nothing publicly that might jeopardize Senate approval of the second Panama treaty.

At the White House the next morning, Torrijos's personal friend and President Carter's principal aide, Hamilton Jordan, placed a phone call to Panama, jordan was angry, and didn't hide it. Panma, he said, must qucikly issue a statement saying it had not decided to reject the treaty the Senate had approved.

Several hours later, the Panamaian government issued such a statement.

But this was not enough. At the highest levels of the American government the feeling grew that there had been a grave lapse in the Panamanian-American relationship. In the delicate days remaining before the Senate's final vote on the second canal treaty, this lapse could prove disatrous unless quickly and effectively dealt with.

Warren M. Christopher, the deputy secretary of state and the man who-with Jordan-led administration efforts on the canal issue, decided on a novel maneuver.

He concluded that the United States needed an intermediary to the panamanians to improve communication between the two governments.

Hoping to recruit such an intermediary, Christopher placed a call to a fellow lawyer and former State Department official, William Rogers, one assistant secretary of state for Latin American affairs, and a partner in the firm of Arnold & Porter.

Ten days earlier, on March 28, Rogers had received another phone call-from panama. Gabriel Lewis, Panama's ambassador to Washington, was on the line. He asked Rogers if he would come to Panama to consult with Torrigos and other officials about the DeConcini reservation.

This small addition to the first treaty asserted an American right to use military force in Panama at any time to keep the canal open and operating. Introducing it, Sen. Dennis DeConcini(D-Ariz.) said it was needed in case, for instance, a labor dispute in Panama closed the waterway.

Rogers checked with the State Department and was encouraged to make the trip, which he did. He found "massive concern" in Panama about the meaning of the reservation, he said in an interview yesterday.

Christopher knew about Roger's trip, and about his good relationship, from previous government service, with Torrigjos and other Panamanians. On that trouble April 7, he asked Rogers if he could volunteer his good offices to serve as a middleman between the United States and Panama. Rogers agreed.

During the next 11 days Rogers became the channel through which every communication passed between Panama City and Washington, Christopher said in an interview yesterday.

The weekend before last, Rogers flew to Florida to meet with one of Torrijos' key ministers. The object of their discussions was to agree on a "bottom line" that Panama required in the way of an addition to the second canal treaty in order to dispel their worries about the DeConcini reservation, according to an administration source.

But Panama's bottom line was-at best-half the administration's problem. At least as serious was whether the Senate could be persuaded to adopt some new language that would assuage Panama's concerns.

Negotiations toward this end were not going well at the beginning of last week. DeConcini was crucial, since all the actors in this drama agreed that he would have to endorse any attempt to reassure Panama about the intent of his earlier reservation. Early last week. DeConcini was scheduled to meet with Carter to try to work something out, but last Monday night both sides agreed that they were too far apart to justify such a meeting.

DeConcini felt strongly that he could not endorse any new treaty language that seemed to nullify or narrow his original reservation. But the Panamanians and the administration proposed language that the freshman senator felt had that effect.

By Thursday of last week Carter personally was fed up. Jordan called a meeting of top officials involved in the Panama fight and let it be known that Carter wanted to go on television that night to say that the Senate ought to adopt a new understanding or reservation to the canal treaty reiterating America's long-standing commitment not to interfere in th e affairs of other countries.

Some participants in this meeting were uneasy about the President's plan. They thought senators would resent instructions from the White House at that delicate moment, and they realized that some senators would feel that Carter was trying to shift the blame for his own mistake-his decision to accept the DeConcini reservation to the first treaty.

Late that day Vice President Mondale and othe administration officials discussed the president's plan on Capitol Hill. The Senate majority leader, Robert C. Byrd (D-W. Va.), the treaties's floor manager, Sen. Frank Church (D-Idaho) and others said it was a bad idea.

The assistant secretary of state for congressional relations, Douglas J. Bennet Jr., a key official in the long Panama Canal dispute, strongly urged that Carter abandon the speech. Late Thursday, Frank Moore, the president's principal lobbyist in Congress, went to the boss and told him not to go through with it. Carter agreed.

Two days earlier Christopher had gone to Byrd with the proposal that some new language be found that could please the Panamanians. Byrd's initial response was cool, according to administration offiicals, apparently because he doubted the seriousness of the political troubles Torrijos faced in Panama as a result of DeConcini's reservation.

Byrd's hesitancy at first discouraged the White house, but by the end of last week his attitude seemed to change. On Thursday and Friday he was telling the administration that the Senate would have to find a solution to the problems raised by the DeConcini reservation.

Byrd had the active cooperation of his majority whip; Alan Cranston (d-Calif.:, Church and Sen. Paul S. Sarbanes (D-Md.), a freshman member of the Foreign Relations Committee who became a leader of the protreaty forces. These four became a team.

DeConcini, meanwhile, decided to try his own hand at drafting a new reservation, since his own he was never satisfied with anyone else's efforts. On Friday he gave Byrd a proposed draft.

On Sunday, Rogers, Ambassador Lewis, Church, Sarbanes and Byrd met in Church's Capitol office. The final drafting was done there, with Byrd adding the last words while hunched over a coffee table on his knees.

DeConcini said yesterday that the final version of the new reservation was very close to his Friday draft, but Rogers and Lewis also felt they had a chance to alter it constructively, and eveyone was apparently satisfied. Lewis said he would have to check it out with Torrijos.

The answer came back in a letter from Torrijos to Byrd. According to a source who read the letter, Torrijos described Byrd as "captain of the ship," and it was up to him to bring it in, Torrijos wrote I'll be sure that the pier is in the right place to meet it.

Christopher remained a party to all the dealings between the Senate leaders, Rogers and the Panamanians, and administration officials reacted angrily yesterday to published comment that the senators had excluded them from the crucial last stages of negotiations.

From the senator's viewpoint, as Cranston noted in an interview, the crucial point was to make sure there would be no misunderstanding this time, and the Roger-Lewid combination provided that assurance.

So on Tuesday the Senate adopted a new reservation, insisting that any action the United States ever takes to keep open the Panama Canal "shall not have as its purpose nor be interpreted as right of intervention" in Panama or "in interference with its political independence or sovereign integrity."

This was enough for the adminstration, Torrijos and 68 senators. The Panama Canal treties were approved.