Sen. Robert C. Byrd (D-W.VA.) declared yesterday that ratification of the Panama Canal treaties would be in the best interests of the United States, and he promised to work for Senate approval.

The Senate majority leader predicted "a difficult battle" in the Senate, but said he was "cautiously optimistic that the treaties will be approved" by the necessary two-thirds majority, provided they are slightly modified.

Byrd's announcement was good news for the Carter administration, which now feels that momentum on the Panama issue is finally moving in its direction. Prospects for Senate approval of a slightly modified version of the treaties appear better now than at any time since the pacts were signed in September.

Byrd's declaration of support for the treaties followed by a week a similar though more cautious statement from Sen. Howard H. Baker Jr. (R-Tenn.), the minority leader in the Senate. Byrd said yesterday he had phoned Baker in Brazil to tell him his decision. Though he declined to speak for Baker, Byrd said he expected to work with him to find a way to add some language to the treaty that both senators support.

The language he has in mind is that of a communique issued Oct. 14 by President Carter and Gen. Omar Torijos, the Panamanian leader. The communique said the new treaties guaranteed the U.S. right to defend the neutrality of the canal with military force, and assured priority passage through the canal to American warships in an emergency.

Byrd said these assurances could be added to the pacts as an amendment, reservation, understanding or something of the kind. The substance, not the label, was the important thing, he said.

Torrijos told Baker in Panama last week that he could accept this language as an addition to the treaties.

Secretary of State Cyrus R. Vance said Thursday that the administration would also accept a reservation or understanding incorporating the communique's language. So all interested parties appear to agree on a change that could substantially enhance the treaties' chances in the Senate.

Opponents of the treaties have promised to introduce dozens of other amendments, many of which the Panamanians have flatly rejected already. Byrd said he would study any amendments that are offered, and he might support some of them.

Byrd declined to predict when the threaties would reach the Senate floor, but said he would bring them up "as soon as I possibly can" after they are approved by the Senate Foreign Relations Committee. Committee action is expected late this month.

The Senate will take a Lincoln's Birthday recess from Feb. 10 to Feb. 20, and the treaties could come up right after that. However, Byrd noted yesterday that this schedule could be upset by a debate on energy legislation, if Senate and House conferees are able to agree on a conference report.

The majority leader predicted tha the treaties will be "hotly debated" for two to five weeks on the senate floor.

Byrd told a press conference yesterday that "the treaties are the best means of assuring continued access to the canal and use of the canal - and that is our primary concern."

He said he had been "impressed by the good faith exhibited by the Panamanians" since the treaties were signed. His trip last year to Panama left him "more hopeful and confident about that country's future political direction." Byrd said. He noted that Torrijos had abrogated "certain repressive laws" in Panama after he and other senators had complained about them.

"Ratification of the treaties would be consistent with our own role as a leader among nations." Byrd said. "It is particularly important for our relations with Latin America and should open a new era of mutual interest and mutual trust and cooperation . . . Given the history of the canal and the principles of our country, the treaties are in our interest, and ratification is the right step to take." CAPTION: Picture, Sen. Robert C. Byrd: "The [Panama Canal] treaties are in our interest . . .", AP