WHO SAYS President Reagan lacks a due concern for human rights? On Tuesday, having let the thing languish for 44 months, he jumped to the support of the United Nations treaty against genocide. It seems the White House had learned that Walter Mondale, an old supporter of the treaty, was about to speak in its favor before B'nai B'rith, the Jewish service organization. The Reagan announcement beat Mr. Mondale to the pass.

Some may find it distasteful that a worthy measure, one fully able to be judged on its own merits, becomes caught up in the play of election-year politics. We say it's cause for gratitude that the genocide treaty gets a further chance to be ratified.

The treaty formally defines genocide as the intentional destruction of a national or religious group, and declares the practice a crime under international law. The measure is commonly and correctly regarded as a symbolic statement in which the nations of the world intended to say that another Holocaust would not be permitted. The man most responsible for its adoption in the United Nations, however, never saw it as exclusively a product of the tragedy of the Jews. As a student in prewar Poland, Raphael Lemkin was stirred by reading of the killing of Christians in ancient Rome. Other, contemporary instances of group persecution drew his attention, and in the 1930s, he lobbied internationally for the drafting of a legal instrument banning mass murder of ethnic groups.

Mr. Lemkin's early efforts failed, William Korey wrote recently in Present Tense magazine, because "the very idea of mass killings was regarded as inconceivable and the subject beyond the pale of international law." Only in 1948 -- after Hitler had made the "inconceivable" real by killing 6 million Jews, as well as millions of others -- did the United Nations adopt, by a 55-0 vote, a treaty banning genocide.

Some 90 nations have since ratified it, but not the United States. All presidents except Eisenhower have supported it. The holdup has been in the Senate, where, in the past at least, a certain legalistic conservative argument has been made that American accession to the treaty would somehow open the United States to being put on trial on a phony pretext by foreign enemies. This strikes us as a dated, not to say farfetched and feeble, argument. Mr. Reagan will surely want to hammer hard against it.

The genocide treaty allows the United States to make a clear moral statement about a frightening practice, which goes on still. The American failure to ratify, moreover, has been a standing embarrassment to this country's protests against gross human rights abuses. The treaty should be ratified, finally, now.