The long-awaited Law of the Sea Treaty moves toward signature this week with the United States absent from the leadership it exercised until the present administration. Washington may be able to start the way back to international cooperation and esteem by assuming a statesmanlike attitude in tomorrow's scheduled US address to the signing conference in Jamaica. It needs, for one thing, to dispel the bad taste of its spoiler efforts to enlist industrial nations in a divisive minitreaty. The latter would defy the document passed by the nations of the world in a 130-4 vote earlier this year.
Throughout the years of negotiation the US has been properly concerned with protecting the interests of American companies in mining the mineral riches of the seabed. Now, by standing apart indefinitely, the US would ironically risk damaging those very interests. The American firms could not mine legally except in compliance with the treaty; they might be forced to operate under the flags of signatory countries. Meanwhile, the US would be in an uncertain position regarding the rest of the treaty, including environmental and economic agreements, not to mention the military rights of passage eagerly sought by the Pentagon.
Might the US actually join the signers, instead of staying on the sidelines with a minority including Britain and West Germany (two which are said to be delaying signing in support of their ally)? What an opportunity this would be for President Reagan to show once more the dramatic flexibility - as on raising taxes and lifting pipeline sanctions - which he has justified to maintain other goals.
Yet, according to reports, the most to be expected of the US is signing of a nonbinding document that would permit it to participate as a nonvoting observer in the Law of the Sea preparatory commission. This body will draft a detailed code for mining the seabed, designated as the ''common heritage of mankind,'' and prepare for setting up the treaty's International Seabed Authority.
It is too bad that the one-time prime mover of the Law of the Sea should deny itself a vote on matters that will affect it on into the future. It must be hoped that the commission nevertheless comes up with provisions that allow Washington an excuse to sign on before too long.