In the Senate, Cooper helped lead the opposition to the development and deployment of anti-ballistic missiles (ABMs), putting him at odds with many in his party, including President Nixon. Cooper had long been an opponent of ABMs, which he believed could intensify a worldwide nuclear arms race. On August 6, 1969, a vote to suspend funding of the development of ABMs failed in the Senate by a vote of 50–51; Vice-President Spiro Agnew cast the tie-breaking vote. After this defeat, Cooper and Michigan Senator Philip Hart co-sponsored the Cooper–Hart Amendment that would have allowed funding for research and development of ABMs, but banned deployment of a U.S. ABM system. The measure failed by three votes but increased congressional scrutiny of the Defense Department budget, leading to a reduction in funding and hastening Strategic Arms Limitation Talks with the Soviets. Cooper served as an advisor to President Nixon during the events leading up to the talks.