The Senate yesterday killed a key amendment to the Panama Canal treaties by a vote of 55 to 34, a margin that heartened administration officials who had feared a closer outcome.

But Sen. James B. Allen (D-Ala.) who introduced the defeated amendment, said the vote was "very favorable . . . from the standpoint of the opponents of the treaties," and promised to press his fight against them.

The defeated amendment would have changed the treaties to allow the president of the United States to maintain troops in Panama until 2019 if he deemed it necessary to maintain the canal's neutrality. As negotiated, the treaties call for final U.S. withdrawal by the end of 1999.

Allen argued that his amendment simply strengthened America's hand in case of any threat to the canal's neutrality after 1999. He said his proposal would supplement the so-called leadership amendments - expected to be adopted later - guaranteeing America's right to defend the canal's neutrality or use the canal expeditiously in any future emergency.

These amendments will be introduced by Sens. Robert C. Byrd (D-W. Va.) and Howard H. Baker Jr. (R-Tenn), the majority and minority leaders, with most of the Senates as co-sponsors.

Treaty supporters said Allen's proposal would be unnecessary because of those leadership amendments, and would be an unacceptable affront to Panamanian sensibilities that Panama would inevitably reject.

The dispute over the significance of yesterday's vote was a variation on the old argument about the half-glass of water - is it half full of half empty?

Allen and his allies took heart from the proponents' failure to get two-thirds of the vote on the motion to "table" (in effect, to kill by ending consideration of) Allen's amendment, and by their success in getting one third of the Senate plus one - 34 votes - one their side.

That is the number of votes the opponents would need to defeat the treaties ultimately.

The Senate leadership and White House both disagreed, arguing that if an amendment like this one that appealed to many Americans' concerns about the future security of the canal could get only 34 votes, then the treaties are in good shape.

Sen. Alan Cranston (D-Calif.), the majority whip, said the vote was "better than we expected." He also said the leadership had demonstrated its confidence by calling for a vote when it knew that a number of its supporters would be absent. Seven announced treaty supporters missed yesterday's vote as did two known opponents and two undecided members.

A White House official involved in the treaty fight said the vote was a big relief, since this amendment seemed to be one of the most politically attractive ones that treaty opponents would offer. "I can't think that they'll come up with one more difficult than that," this official said.

An aide to minority leader Baker expressed a different view, arguing that more subtle but potential ruinous amendments may be offered in the future that could win more votes.

Allen said after the vote that it "indicates that senators are becoming more independent regarding these treaties. They do want to see us get the strongest possible treaties."

Baker and Byrd are working to avoid approval of any amendments or reservations that could jeopardize acceptance of the final treaties in Panama.

Before the vote yesterday, Byrd told reporters he perceived a "gradual but continuing momentum" toward support of the treaties.

Whatever the outcome, Senate leaders predict it will not be final anytime soon. Cranston said it would be difficult to conclude action on the treaties before the end of March.