President Clinton urged the Senate yesterday to delay voting on a global treaty to ban nuclear testing, warning that its virtually certain defeat would give a "green light to every other country in the world" to test, develop and modernize nuclear weapons.

Clinton's appeal came as the Senate formally opened debate on the Comprehensive Test Ban Treaty, and its leaders engaged in a test of wills over efforts planned by Democrats to avert a vote on the pact scheduled for Tuesday or Wednesday.

Senate Majority Leader Trent Lott (R-Miss.), who has demanded that Clinton also agree to put off further action on the treaty until after next year's elections, said he is open to further initiatives by the president. But for now, Lott said, the vote is still on for next week.

At a news conference in Ottawa, where he was conferring with Canadian Prime Minister Jean Chretien, Clinton said he was asking the Senate to put off action because "we don't have the votes," a reference to head counts that show the treaty as much as 20 votes short of the two-thirds needed for ratification.

In what appeared to be an oblique nod toward Lott, Clinton said that the treaty would not be brought up again "until we think we can pass it" and that it ought to be "taken out of politics." But aides said this did not satisfy Lott, who wants a more ironclad guarantee that Clinton not press the issue for the remainder of his term. Democrats say Clinton should not, and will not, give such assurances.

As debate started yesterday, Democrats said that if Lott does not move first they will employ an unusual parliamentary tactic aimed at cutting off consideration of the treaty and moving the Senate back to regular legislative business at some point before the vote begins.

To do so would require a majority vote. Suggesting that it was "pigheaded" to go ahead with the vote, Minority Leader Thomas A. Daschle (D-S.D.) noted that many Republicans have urged a delay and expressed guarded optimism that he could pick up enough GOP support for the 45-member Democratic minority to prevail on the issue.

But Lott appeared ready to fight against any GOP defections. His spokesman, John Czwartacki, said it would be "unthinkable . . . a direct assault at majority control" for the Democrats to try to usurp Lott's scheduling power.

In the day-long opening debate, several senators joined in calling for a delay. "Have we totally lost all sense of responsibility? . . . Why do we have to do it now?" asked Sen. Robert C. Byrd (D-W.Va.). "We are trapped in a political swamp," complained Sen. Chuck Hagel (R-Neb.). "Don't let us get into a box we can't get out of. . . . This is not responsible government."

The debate was sparsely attended by senators and tended to track over ground already deeply furrowed in three days of hearings that were hastily called after Republican leaders decided abruptly last week to move ahead with a treaty vote.

"The treaty will jeopardize rather than enhance our national security," said Sen. Jon Kyl (R-Ariz.) in summing up the case for treaty foes. "It will undermine our vital nuclear deterrent by jeopardizing the reliability of our nuclear stockpile. It will prevent us from making our weapons as safe as they can be. It will not help stop nuclear proliferation and is not verifiable."

Treaty backers disagreed and said rejection of the pact would pose far greater dangers to this country as well as the rest of the world. "If we do not ratify this treaty, we will miss a historic opportunity to stem the tide of nuclear proliferation," said Sen. Carl M. Levin (Mich.), ranking Democrat on the Armed Services Committee. "We will instead be encouraging a new and possibly worldwide nuclear arms race."

Foes said technology is insufficient for maintenance and modernization of the U.S. nuclear stockpile without tests and for monitoring low-level explosions by other countries. "Militarily significant" tests can be conducted "with little or no risk of detection," said Sen. Wayne Allard (R-Colo.).

If the treaty is so flawed, then why are America's closest allies so "apoplectic" about reports that the treaty is likely to be rejected by the Senate, asked Sen. Joseph R. Biden Jr. (D-Del.), referring to rare appeals this week from European leaders for the Senate to act favorably on the treaty. The treaty would protect this country's nuclear superiority while "considerably restrict[ing] the ability of other nations to threaten this U.S. superiority," he said, and low-level tests are unlikely to have much effect on the nuclear balance.

During the debate, Lott urged rejection of the treaty and said Republicans were only doing what Clinton wanted when they agreed to bring up the treaty. "I'm ready to vote," he added.