LAWMAKERS have breathed tentative new life into a treaty that would slash stockpiles of American and Russian nuclear weapons. A decision by the Senate last week to eliminate a date-certain for the deployment of a controversial US ballistic missile defense system lowers the major obstacle to Russian ratification of the 1993 Strategic Arms Reduction Talks (START) II agreement. But the main barrier to US ratification - a contentious dispute between Congress and the White House over reorganizing the State Department - remains in place. At stake is a treaty that supporters say is crucial to global efforts to reduce the threat of nuclear proliferation. (Congress takes up defense budget, Page 3.) ''The more warheads that exist, the more likely it is that one of them will make it into the hands of a rogue nation,'' says Sen. Carl Levin (D) of Michigan. Senator Levin led a successful effort last week to delete a provision mandating deployment by 2003 of a multi-site national missile defense system from a $265 million defense authorization bill. Actual deployment - as opposed to the mere development of Antiballistic Missile (ABM) technology, which the legislation does call for - would put the US in violation of the 1972 ABM Treaty and thus risk rejection of START II in the Russian parliament, supporters of the amendment argued. Though the new language hardly ensures ratification by the Russian Duma, one senior administration describes it as a ''vast improvement.'' ''It will avoid a threat to ratification in the Duma and hopefully give [the treaty] a boost,'' adds Levin. A House-Senate conference committee could reverse the Senate's action this week by mandating a date for ABM deployment. If that threat to START II is averted, attention will then turn to Senate Foreign Relations Committee chairman Jesse Helms (R) of North Carolina, who supports the treaty but has promised to hold it in committee until the dispute over a plan to reorganize the nation's foreign-policy bureaucracy is resolved. A 1993 convention that would ban arsenals of chemical weapons is also on hold. ''Everything has to await the resolution of the State Department reorganization issue,'' says one congressional source. ''To say that we should jeopardize START II for the sake of a domestic internal argument about foreign-affairs agencies is not an appropriate way to handle an important treaty,'' rejoins John Holum, head of the United States Arms Control and Disarmament Agency. Russia roiled Congress's sudden enthusiasm for ABM defenses, combined with the Clinton administration's determination to expand NATO to include several former Soviet bloc nations, has strained relations between Moscow and Washington and even threatened conventional-force agreements between the two nations. Last month, a spokesman for Russia's foreign ministry predicted that any US violation of the ABM Treaty would prompt the Duma to ''refuse to ratify Start II and might lead Russia to withdraw from the START I [treaty].'' The ABM Treaty limits the United States and Russia to one defensive missile site each, thus preserving the vulnerability to a retaliatory strike that has been the main deterrent to starting a nuclear war. ''There's always been a relationship between strategic-offensive-force reductions and limits on missile-defense systems. The relationship is that neither side would have the confidence to dramatically reduce their offensive forces if they felt they would have to face defenses on the other side,'' says Jack Mendelsohn, deputy director of the private Arms Control Association, a think tank in Washington. The treaty will not take effect until the instruments of ratification are formally exchanged, presumably at some future summit meeting between US and Russian leaders. If the Duma does ratify START II, it is likely to issue a caveat stating in effect, in Mr. Mendelsohn's words, ''that the treaty is good only as long as ABM is good.'' The START II treaty was signed by President George Bush and Russian President Boris Yeltsin in January 1993. It mandates the elimination of ''heavy'' and multiple-warheaded intercontinental ballistic missiles. Together with START I, which came into effect last December, it would reduce US and Russian nuclear arsenals by two-thirds. Helms plan waylays treaty Senator Helms has proposed and President Clinton has resisted eliminating and folding three foreign-affairs agencies into the State Department: the Arms Control and Disarmament Agency, the Agency for International Development, and the US Information Agency. The two met last month to resolve their differences on the matter. One GOP Senate source says the ball is now in the administration's court to either accept the Helms plan or come up with a counterproposal that will reduce the State Department's annual budget by the $5 million mandated by the Senate Budget Committee. Clinton administration officials say a vice presidential task force has produced just such a proposal, one that would save the three agencies but still cut thousands of jobs from the foreign-policy bureaucracy. START II is likely to remain in limbo until the impasse is resolved. If and when it is, START II will be voted out of Helms's committee and sent to the Senate floor, where ratification is all but ensured.