The reaffirmation of U.S. rights to defend the Panama Canal has saved President Carter's canal treaties from certain defeat in the Senate, but their approval is still by no means assured, Senate Majority Leader Robert Byrd (D-W.Va.) said yesterday.

Byrd described a joint statement of understanding issued by Carter and Panama's leader, Gen. Omar Torrijos as a "very important diplomatic achievement" and said it would allay many fears about the treaties.

But he warned that "the great majority" of senators, including himself are still uncommitted and that the President has a tough selling job ahead.

"Without the statement, I'm sure the treaty would not have been ratified," the West Virginia Democrat told his weekly press conference on Capitol HIll. "With the statement, I think its chances are improved."

In an agreement clearly aimed at quelling criticism in both the United States and Panama, the two leaders Friday reaffirmed the U.S. right to use military force to keep the canal open and retain "head-of-the-line" canal privileges in times of emergency. They also stated that these rights do not permit the United States to interfere in "the internal affairs of Panama."

One critic, Sen. Bob Dole (R-Kan.), yesterday called the Carter-Torrijos statement "a step in the right direction" and said he intends to incorporate it into two amendments he will introduce on Monday.

"Since the leaders of both nations have agreed upon this more specific language, it should be incorporated into the treaty itself" he said in a prepared statement. "There is no longer any justification for leaving the treaty language ambiguous."

But Nevada Sen. Paul Laxalt, whose Republican treaty opponents have names as their spokesman, said the agreement "will satisfy no one. It doesn't reach the basic issue of why we should give up the canal at all.

"In addition, those who want the entire agreement within the four corners of the treaty won't feel bound of a verbal understanding of two leaders who are both in serious trouble in their own countries on this issue."

Most of the immediate reaction to the Carter-Torrijos agreement appeared positive, however, Alabama's John Sparkman, chairman of the Senate Foreign Relations Committee, called the statement be "a fine step forward" to resolving the doubts of Senate critics Sen. Clifford P. Case (N.J.), the committee's ranking Republican, said the agreement "is a welcome effort to clarify the situation."

Carter and Torrijos hammered these agreement out in a hastily arranged, 100-minute meeting at the White House Friday, three days after a group of senators had warned the President that the Panama Canal treatiess had no chance of passage in the Senate.

Both leaders were under pressure in Panama, which will hold a treaty plesbiscite Oct. 23, military ruler Torrijos was under fire from those who felt the treaty would give the United States rights to intervene in domestic affairs. Here, statements by Panamanian officials implying that the treaties did not give the United States to protect the canal or obtain priority passage for its ships in war caused widespread concern.

Byrd yesterday said Carter "was wise to move quickly" and the agreement was "definitely a plus." He gave little indication of how much help it would be, however. "Obviously, the statement won't lose any votes. It can only gain votes for the treaty," he said.

Byrd indicated some concern about reports linking Torrijos with illegal narcotics traffic. Torrijos brotherre, Moises, now Panama's ambassador to Spain, was indicted in absentia in New York five years ago for drug trafficking.

But the Senate Majority Leader said Attorney General Griffin Bell has told key senators in briefings that Gen. Torrijos himself was not personally involved.