Facing an embarrassing foreign policy defeat for President Clinton, Democrats yesterday mounted a last-ditch effort to persuade the Senate's Republican majority to delay a vote scheduled for today on a treaty banning nuclear tests.

But the effort ran into objections from some Republicans, and Majority Leader Trent Lott (R-Miss.) said that negotiations over delaying the vote will continue today. Several senators said they expect an agreement to call it off, but said an objection from even one senator could derail it.

"We don't have an agreement at this point to do anything but go forward with the vote, but we are exploring all kinds of possibilities," Lott told reporters. "You keep looking and seeing if there is language you can get agreement on. If you can't, then . . . you vote."

If compromise efforts fail, the Comprehensive Test Ban Treaty is scheduled to come up for a vote late today. Treaty supporters have nowhere near the two-thirds majority of 67 votes needed for ratification of a treaty. Only two of 55 Republicans have so far announced their support for it.

Earlier in the day, it appeared the two sides were nearing agreement to delay the vote after Minority Leader Thomas A. Daschle (D-S.D.) pledged in writing to Lott that he would not press for a vote on the pact until after next year's elections, barring an unforeseen international emergency.

Republicans had demanded that Clinton pledge not to bring the treaty up again, which he has refused to do, and several, including Lott, said they wanted an "absolute" commitment that there will be no vote next year. But others said Daschle's proposal went a long way toward satisfying their demands.

According to several sources, Lott and Daschle agreed to float a proposed compromise among their colleagues under which treaty hearings would be held next year but it would not be brought up for a vote until after the elections--barring "extraordinary circumstances," as Democrats described the language. Meanwhile, Sens. John W. Warner (R-Va.) and Daniel Patrick Moynihan (D-N.Y.) launched a separate effort to rally support for a delay.

Democratic spokesman Ranit Schmelzer said Democrats agreed to the compromise. But Lott confirmed that several Republicans, including Foreign Relations Committee Chairman Jesse Helms (N.C.) and Sen. Jon Kyl (Ariz.), had problems with it. Under procedures being used by Lott and Daschle, every senator must agree to cancel the vote.

An earlier Democratic plan to use a parliamentary maneuver to postpone action on the treaty by majority vote collapsed when Republicans agreed to stick together in opposing it as an assault on Lott's scheduling prerogatives. Democrats would have failed on a party-line vote of 55-45.

A senior White House official said Clinton will not intervene if Senate Democrats promise to keep the treaty off the floor during the remainder of his term, if that's what it takes to avoid a vote against ratification. "I think this is essentially a matter between the majority leader and the minority leader," said the official. The official said Clinton wants the freedom to continue speaking on behalf of the treaty.

The treaty, which would ban all nuclear testing by signers and require monitoring, inspections and sanctions against violators, has long been one of Clinton's top foreign policy priorities. But, when Democrats began pushing for a vote this fall, Republican leaders, confident of being able to defeat the treaty, called their bluff and scheduled action on the pact for this week. As the vote neared, however, many in both parties began to worry about the international fallout from a U.S. rejection.

The treaty has been signed by 154 countries but ratified by only 51, including 26 of the 44 nuclear-capable countries that must act before the treaty can go into effect.

Arguments over whether to postpone the vote dominated argument on the Senate floor as senators engaged in a second day of debate over the treaty itself, disagreeing over whether a prompt rejection of the pact would send a message of strength or weakness to the world.

The treaty "will not improve with age," said Sen. Jon Kyl (R-Ariz.) arguing that a emphatic vote to reject it would make it clear that the United States demands "minimum standards" for its own security and compliance by other nations that are not met by this treaty.

But Sen. Arlen Specter (R-Pa.), one of only two announced Republican supporters of the treaty, argued that rejection would encourage such countries as India and Pakistan and perhaps others including Libya, Iraq and Iran to test weapons with a view toward development of a nuclear arsenal.

Others bemoaned the political gamesmanship that contributed to the impasse. "Something unusual and unsettling has happened to our politics when party lines divide us so clearly and totally on a matter such as this," said Sen. Joseph I. Lieberman (D-Conn.). "That's not the way it used to be in the United States Senate and it's not the way it should be."

Staff writer Charles Babington contributed to this report.