PRESIDENT Barack Obama and Russian President Dmitry Medvedev have made reaching a new arms agreement the centerpiece of efforts to revive US-Russian relations. But they may face an impossible deadline for getting the deal in place. Missing that goal would be diplomatically uncomfortable, as the negotiations will be the first test of how well Moscow and Washington can work together on complex issues and roll back a product of Cold War animosity – thousands of nuclear warheads ready for launch. Last week in London, Obama and Medvedev pledged cooperation after years of mistrust. As a first step, they said they would seek to replace the 1991 Strategic Arms Reduction Treaty (START) before it expires in December. But negotiators, who have not yet begun, will be hard pressed to meet that goal. The Obama administration knows that they will need not only on good faith from Moscow but also from some Republicans in the US Senate, who Obama would need to ratify an agreement with the necessary two-thirds majority. Some Republican lawmakers oppose a new treaty. Republican Senator Richard Lugar, a senior statesman among US arms control advocates who is supportive, said lawmakers would need a document agreed to by Moscow and Washington by August to consider it and ratify it by December. “That means it's not an eight month process but four or five,” he told The Associated Press. The new treaty would be the first step of an ambitious arms control agenda for the Obama administration that will require Moscow's support to get anywhere. Last week in Europe, Obama pledged to work toward ridding the world of nuclear weapons. He also wants to rebuild the world's fraying nonproliferation institutions. A new START treaty appears to be achievable, because both sides want to replace the old one and seem to agree on some of the broad goals. But negotiators will be challenged by years of mistrust between the two countries. As Russian ambassador to Washington Sergey Kislyak put it Tuesday, the two countries have a long history of cooperation on reducing their arsenals, but have made little progress in recent years. “We have lost a culture of negotiating together seriously,” he said at an event sponsored by the Carnegie Endowment for International Peace, a think tank. The 1991 START treaty was a complex document. It capped warheads and ways of delivering them and detailed extensive procedures for verifying disarmament that have been a basis for cooperation on disposing nuclear materials. Both sides want to simplify some of the verification procedures and reduce warheads. The Obama administration appears ready also to meet Russian demands of also reducing delivery systems (bombers, missiles and submarines), something that the Bush administration opposed. As it stands now, the United States has 2,200 strategic nuclear warheads deployed; Russia has 2,800. The two sides agreed to further warhead cuts in 2002, and Russian and American arms control experts believe that the START replacement treaty would seek to cut arsenals to 1,500 on each side. But there are extensive details to work out. Complicating matters, US and Russian negotiators have not yet begun. The senior US negotiator Rose Gottemoeller was only just confirmed in her State Department post and was preparing her team to leave quickly for talks with Russian counterparts. Before her new posting, Gottemoeller was pessimistic about the two sides meeting the deadline, writing in an October paper for the Carnegie Endowment, where she worked until recently as head of the Moscow office, that chances of ratifying a new agreement before the old one expires was “extremely unlikely.” But that was at the height of tensions during the last months of the Bush administration, which had resisted a detailed agreement to replace START. Speaking in her first public event as a member of the Obama administration, on a panel with Kislyak she had a new assesment. “This is a difficult task, but it's a doable task,” she said.