Though greatly relieved by their apparent success in winning senatorial support for the Panama Canal treaties, both the White House and the Senate leadership remain too nervous to claim victory.

The best available head count now suggest that a vote next week on the treaties would produce the necessary 67 votes in favor. But the vote will not come next week. It may not come before April.

And in the interim, the treaties' sponsors fear, a great deal could go wrong. Proof of that came Friday when the Senate decided to hold a secret session after the current recess to hear charges that Gen. Omar Torrijos, Panama's "maximum chief," of members of his family are involved in narcotics smuggling.

The idea for a secret session came from Sen. Bob Dole (R-Kan.), who has been making political hay with the treaties for weeks. Privately, administration officials deplored his move as cheap politics, but they also acknowledged that the mere calling of a secret session for Feb. 21 raised the sort of public doubt and confusion they had hoped to avoid.

An even greater threat to the treaties appears to be the possibility that a majority of senators will support some amendment or reservation that will force Panama or President Carter to reject them, or at least require a new plebiscite in Panama.

Several key treaty opponents in the Senate now agree that they are more likely to block or subject the pacts through the amendment process than by finding 34 senators to vote against the approval (one-third plus one of the 100 senators).

Sen. James B. Allen (D-Ala.), the Senate's most resourceful and ingenious parliamentarian, has already spoken of "dozens of substantive amendments" that the Senate should consider once it begins article-by-article consideration of the treaties.

In the office of Sen. Howard H. Laker Jr. (R-Tenn.), an important treaty supporter, one aide believes he has already dreamed up a "killer amendment" that could scuttle the treaties because a majority of the Senate could not resist it but the Panamanian government could not accept it. What would such an amendment say? Baker's aide declined to answer.

A number of possible amendments that might play the "killer" role are under discussion either among treaty opponents or administration lobbyists trying to second-guess the opponents.

A simple majority of senators can approve an amendment, and the treaties' backers are not certain they can persuade a majority to vote against some proposals that may be attractive by American standards, though repugnant to Panama.

For example, the treaties permit the United States to station troops in Panama until 2000, and commit the Panamanians to keep all foreign troops out of their country thereafter. The treaties also provide that either the United States or Panama can act to defend the neutrality of the canal after 2000.

Treaty opponents feel this is too vauge. They might propose an amendment giving the United States the right to maintain troops in Panama after 2000 if it perceives a need to do so. Such an amendment could be worded innocuously, as though the United States does not now expect to keep troops in Panama after 2000, but wants to preserve the legal right to do so against some unforeseen eventuality.

Proponents of such an amendment could argue that it would be foolish to decide in 1978 that the United States must take its troops out of Panama in 22 years, regardless of what may happen in the iterim. Let us pledge our good intentions but keep open our options, they could argue. That plea would appeal to a large number of senators.

But it would probably infuriate the Panamanians, for preserving that American "right" would amount to a prolonged denial of complete Panamanian sovereignty in Panama.

A variation of the same idea might involve in amendment to the treaty provision pledging the United States not to interfere in Panama's internal affairs.

What would happen, Allen asked in an interview last week, if the treat to the canal's neutrality came from within Panama? Could the United States act without "interfering" in Panamanian affairs? Allen thought not, and indicated he will propose dropping the language from the treaty forbidding such interference.

Here again an amendment might be drafted innocuously, not asserting an American right to intervene in Panama, only eliminating the prohibition of such intervention.

But such an "innocuous" change could easily cause a furor in sensitive Panama.

Allen and other treaty opponents also complain that the idea of a "neutral" canal does not really serve U.S. interests. What would happen, an aide to Allen asked in an interview, if the United States got into a land war in Europe with the Soviet Union, and Soviet submarines tried to use the canal during such a war? Wouldn't the United States want the legal right to prevent that?

Here again an amendment could be offered giving the United States an effective veto over whose ships could use the canal in certain emergency situations. It could be sold to the Senate as just good sense, but again could infuriate the proud Panamanians.

Allen charged that the policy in Washington now is "clear it with Omar" - a reference to the New Deal slogan. "Clear it with Sidney" (Sidney Hillman) that symbolized the power of organized labor in Franklin D. Roosevelt's administration. This infuriates Allen and some others in the Senate, who may seek a way to force their colleagues to vote on an amendment whose only real purpose is to compel Torrijos to hold another plebiscite in Panama - a demonstration of senatorial independence.

The administration fears that Torrijos could lose a plebiscite, or demand changes of his own in the treaties if the Senate forces changes.

One reservation or understanding that now seems likely to pass the Senate - and which the administration can probably accept - involves potential U.S. liability if the new arrangements for running the Panama Canal under the treaties prove unprofitable. The Senate seems inclined to insist that the U.S. Treasury not be liable to make up for insufficient canal revenues.

The biggest danger to the treaties may be an unforeseen issue that comes up during debate. "We have to be concerned about unexpected issues." Majority Whip Alan Cranston (D-Calif.) said in an interview last week. "The opponents keep finding new issues."

Another serious threat to the pacts is the position taken by Sen. Robert P. Griffin (R-Mich.), the opponents' floor leader for the canal debate. Griffin argues that the Senate should give its "advice" to President Carter, but not its "consent."

"Send it back for more negotiations," he suggests.

At the right moment in a tense debate - perhaps a debate on one of the possible amendments - a motion to that effect might just take the Senate by storm.

So while members of the Senate are at home next week - hearing about the canal treaties from their constituents one last time before casting final votes - the White House and the Senate leadership have lots to worry about.