Though the debate on the Panama Canal treaties in the Senate has barely begun one of the closest and most important votes in the entire procedure could take place late today.

That will be a vote on the amendment proposed by Sen. James B. Allen (D. Ala.), perhaps the most determined and ingenious opponent of the treaties in the Senate. Allen's amendment would allow the United States to maintain armed forces in Panama until the year 2019 -- 20 years longer than the treaty as negotiated would permit.

White House lobbyists say they are afraid of this amendment, because the U.S. right to defend the canal has consistently been a source of concern to numerous senators.

One White House official predicted flatly that the Allen amendment would win well over one third of the Senate votes cast. In other words, well under two-thirds would vote against it. This should not be taken as a sign that the final vote on the treaties would fall below the needed two-thirds, the official said.

"A lot of these amendments will be very attractive," he added, predicting that a number of them may be defeated by narrow margins, or even approved. The treaties' supporters hope to avoid any changes beyond the two amendments proposed by the majority and minority leaders, which have already been accepted by Panama and President Carter.

Majority Whip Alan Cranston (D. Calif.) said in an interview Friday that he would be happy to get 51 votes against the Allen amendment, though he expected to get "a few more than that."

Allen says he doubts the accuracy of this prediction. He said Friday night in a telephone interview that he doesn't expect more than 35 votes in favor of his amendment.

In fact, both sides may be exaggerating their opponents' strength so they will be able to boast about the result tonight.

Administration officials note hopefully that Allen may be premature in bringing up what might be his most attractive amendment at the outset of debate.

Allen insisted that he hopes to win 34 votes against the treaties -- enough to deny two-thirds approval -- by illustrating their weaknesses in debate on a series of amendments that he and other opponents say they intend to introduce. He suggested that this might be a successful tactic even if none of the amendments he offers are adopted.

It will take a simple majority of senators to adopt an amendment to one of the two treaties, whereas two-thirds are needed for final approval.

Senate leaders and the White House are afraid of amendments that would force renogotiations with Panama or a new plebiscite there to ratify an altered pact.

Majority Leader Robert C. Byrd (d. W. Va.) warned Friday that the Allen proposal was "a killer amendment" because it would "kill these treaties" if adopted.

Byrd and others who back the treaties argue that prolonging the presence of American troops in Panama would be an affront to the Panamanians and is not necessary in light of other provisions in the treaties to assure the United States the right to defend the canal's neutrality after the year 2000.

Byrd met with White House strategists Friday to plan for today's vote, which will probably come late in the afternoon.