President Clinton, suddenly given a make-or-break chance to achieve one of his administration's top priorities, yesterday launched a full-scale campaign to ratify the Comprehensive Test Ban Treaty in the face of extensive Senate opposition.

Clinton, who had dispatched the vice president and top aides to speak for the treaty over the weekend, personally took charge of the lobbying effort yesterday, raising the stakes for an issue likely to be recorded as a major accomplishment or keen defeat for his presidency. He spoke to reporters, assembled his national security team and said he will speak individually and in small groups with senators whose minds he needs to change if he is to gather the two-thirds majority needed when the treaty comes up for a ratification vote next week.

"This is very important to protecting our people from the danger of nuclear war," the president said at the White House. "It would be, in my judgment, a grave mistake not to ratify the treaty."

Much is riding on the outcome of next week's vote in both political and military circles. Clinton, like previous presidents, argues that the treaty would preserve U.S. nuclear superiority and prevent proliferation in other countries. A Senate rejection would mark the first defeat of a major treaty proposal since the end of World War I, signaling a new willingness by Congress to challenge the executive branch on arms control matters in the wake of the Cold War's thaw.

"A victory will be an important part of [Clinton's] positive legacy; a defeat will be an important part of his negative legacy," said John Isaacs, president of the Council for a Livable World, a prominent advocate of the treaty. "It's a long shot."

Clinton acknowledged that he lacks enough votes in the Senate right now. While their leaders say all 45 Democrats support the treaty, only seven Republicans have come out in favor, leaving the White House 15 votes shy of the number it needs.

The White House scored a small tactical victory yesterday when Senate Foreign Relations Committee Chairman Jesse Helms (R-N.C.)--who had long refused to hold hearings on the treaty--reluctantly scheduled one for Thursday, with Secretary of State Madeleine K. Albright among the witnesses. The Armed Services Committee will open hearings today.

But Helms and other Senate GOP leaders made clear their antagonism toward the treaty, accusing the administration of disinformation and arguing that the pact would prevent the United States from modernizing its nuclear arsenal.

"The Comprehensive Test Ban Treaty is dangerous and should be defeated when it comes to a vote in the Senate next Tuesday," said Senate Majority Leader Trent Lott (R-Miss.).

The 1996 treaty has been signed by 154 nations, including the United States, but the U.S. Senate never ratified it. Only 47 nations have ratified it. Of the 44 nations that are capable of making nuclear weapons and whose approval is needed for the treaty to take effect, only 23 have ratified.

Russia and China are among those who haven't ratified it, and the White House said yesterday they are looking to the United States for leadership on the matter.

Treaty proponents argue that the pact would lock in the nuclear superiority of the United States, which has the world's largest nuclear arsenal and stopped testing in 1992, in part because it can simulate tests with powerful computers. Opponents say the treaty would bar the United States from updating its nuclear weaponry while enemy states might secretly test weapons undetected.

Senate Armed Services Committee Chairman John W. Warner (R-Va.) said a test ban would prevent the United States from testing the reliability of its nuclear arsenal. The treaty would thereby "induce the leader of another nation or some rogue element or some terrorists to challenge the United States." He said he was also concerned about the CIA's inability to monitor low-level nuclear tests by Russia.

The CIA recently concluded that it cannot monitor Russia's low-level nuclear tests precisely enough to ensure compliance with the treaty. Clinton yesterday dismissed such concerns, saying very low-level tests "are hard to pick up now [and] they'll be hard to pick up if this treaty is ratified. If this treaty is ratified, there are new tools to monitor the testing of others." The treaty calls for 321 monitoring sites and on-site visits by inspectors.

"Since we don't need nuclear tests, it is strongly in our interest to achieve agreement that can help prevent other countries, like India, Pakistan, Russia, China, Iran and others, from testing and deploying nuclear weapons," Clinton said. "The treaty will also strengthen our ability to monitor if other countries are engaged in suspicious activities."

Staff writer Helen Dewar contributed to this report.

CAPTION: With Defense Secretary Cohen, Joint Chiefs Chairman Shelton and Energy Secretary Richardson, President Clinton makes pitch for treaty.