The talk of the U.S. Senate these days is SALT II - jillions of words of talk - but it can't quite hold a candle to the oratorical marathon of another treaty.

That treaty, committing the United State to formal opposition to genocide, has been pending before the Senate since June 16, 1949 - more than 30 years.

And almost every day that the Senate has been in session since 1967, Sen. William Proxmire (D-Wis.) has taken the floor to speak briefly in support of the genocide convention.

Unlike other legislative matters, treaties submitted to the Senate for ratification do not die when a Congress adjourns. Thus the genocide convention ranks as perhaps the most ancient pending item on the calendar.

Since Harry Truman, every president and secretary of state has supported ratification of the genocide treaty, yet it lingers on the calendar in what Proximire describes as "a real mystery."

The mystery is such a mix of politics and clashing philosophy that Proxmire, the major domo of the ratification effort, thinks that 1981 is a "realistic" target for finally getting it passed.

Curious commentary, Proxmire noted, for a nation that waves the flag of human rights and thinks of itself as a paragon of international morality and decency.

The U.N. General Assembly, reacting to the atrocities of Nazi Germany, passed a resolution in 1946 declaring genocide a crime under international law. Two years later it voted, 55 to 0, to approve the text of the convention.

The treaty is aimed at preventing the destruction of national, ethnic, racial or religious groups by defining and outlawing genocide, and setting procedures for trying and punishing violators.

President Truman sent the treaty to the Senate with strong urging that it be ratified. President Carter, like others before him, has urged ratification. In May 1977, he told the United Nations that he would work for Senate ratification.

But other things - notably the strategic arms limitation treaty - have gotten in the Senate's way and the genocide convention languishes.

Not that Proxmire hasn't tried to keep the subject alive. He has missed making his daily statement only when he was away from the Senate a few days, usually back in Wisconsin when no rollcall votes were scheduled.

"It is really a mystery to me, why we can't get this ratified," he said the other day. "For years, we had the opposition of the American Bar Association and Sen. Sam J. Ervin (D-N.C.), which made it very tough."

"But the ABA now supports the treaty and Sen. Ervin has retired," he added. "The only opposition comes from the Liberty Lobby, the John Birch Society and some far-out groups of that sort."

Proxmire said his zeal for the treaty derives from his idea that "it is the basic human rights treaty...There's nothing more sacred than human life, and the notion that any group can be exterminated is shocking. The treaty helps overcome the notion that nations are a law unto themselves."

Actually, the notion is pretty universally rejected. Eighty-three nations have ratified the convention, leaving the United States and China as the only exceptions among the major countries.

The Senate Foreigh Relations Committee has held four sets of hearings on the treaty and four times has recommended ratification. Minority opposition kept it from floor debate three times. The fourth time it reached the floor, in 1974, and was promptly filibustered by Ervin.

Two attempts to cut off the filibuster failed to get the required two-thirds vote, and the pro-treaty forces threw in the towel. After Ervin retired, the late Sen. James Allen (D-Ala.) took up the mantle as opposition leader.

A successor-apparent to Allen is Sen. Jesse Hels (R.-N.C.), who has indicated he would try to talk the treaty to death. But Proxmire thinks he can see a light at the end of this 30-year tunnel.

"The television showing of "Holocaust" was a great boon to us, creating an emotional feeling about genocide, reminding people," Proxmire said. "I have found virtually no one in the Senate interested in blocking the treaty, but the leadership fears that if it comes up now, the opponents of SALT might cause delays on both treaties."

That means there will be no U.S. ratification of the genocide convention in 1979. What about 1980? Well, Proxmire said, some senators have said they would support the treaty if it comes up in a non-election year - which means that 1980 doesn't look good.

"I think we're making real progress," Proxmire added. "Now 1981 seems to be a realistic target for ratification."

For a long-distance runner such as Proxmire, who can jog and talk for miles on end, that's not far away at all. CAPTION: Picture, Treaty advocate William Proxmire displays 1949 message from President Truman. By James K. W. Atherton - The Washington Post