That plan, like so many of the splendid diplomatic plans -- on global warming, on fostering "a new beginning" in the Middle East -- with which Obama arrived in office in what now feels like a different century, has turned to dust. But while in those other cases the failure lay either with the administration's own expectations or with the recalcitrance of other countries, the nonproliferation agenda has been disabled by domestic opposition. Yes, the Russians haggled endlessly, but they did finally agree to a treaty that offered meaningful mutual cuts and, most importantly, ensured that both sides would be able to verify that those cuts were in fact being made. Obama won important commitments on his core issue of stopping nuclear proliferation at the U.N. Security Council in September 2009 and again at the NPT review conference in May. The stumbling block on all these issues has been the Republican caucus of the U.S. Senate -- and the opposition will only grow when the new Senate is seated. That body, both more Republican and more conservative, almost certainly will not approve the CTBT or deeper arms reductions with Russia.

Obama doesn't need Senate approval to achieve all his nonproliferation goals. He will keep the pressure on Iran through a combination of diplomacy and sanctions. He can strengthen the provisions of the NPT with regard to weapons inspections and punishing states, like North Korea, that withdraw from the treaty. He can work on developing an international nuclear fuel bank so that states do not need to develop their own fuel cycle to produce enriched uranium for peaceful purposes. He can press his NATO allies to reduce the organization's reliance on nuclear weapons, an issue that will come up at this weekend's meeting in Lisbon as NATO begins to rewrite its strategic concept.

But the problem is that, as with health-care reform, practically everything depends on everything else. The basic bargain of the NPT is that nuclear-weapons states will move toward disarmament while other states will not acquire such weapons and will work to prevent proliferation. The United States has to do the one in order to get the other. As I mentioned in an earlier column, Senator Kyl has told me that he thinks it's naive to imagine that other states will stick to such a bargain. Obama believes, and virtually every expert and diplomat who has worked on these issues agrees, that there's no other way of persuading states to take proliferation issues seriously. Obama's success at the United Nations and at the NPT conference had everything to do with his commitment to change the direction of U.S. nuclear policy. For that reason, the very fragile international consensus on Iran may not hold if the United States is seen as flouting the rules.

Looking further down the road, the only way to bring Britain, China, France, and perhaps even India, Israel, and Pakistan into what is now a two-country arms control regime is to reduce nuclear stockpiles deeply enough that other states feel safe in building down as well, and in relying instead on conventional capacities (and perhaps missile defense). The United States and Russia will have to agree to go below 1,000 warheads -- still quite enough to obliterate the world many times over. But this can't happen if Republicans keep behaving as if Russia is the Soviet Union circa 1953 -- or worse still, as if denying Obama a victory is a fair price to pay for increasing the likelihood that terrorists get their hands on a nuclear weapon. If that thought doesn't move America's even-keeled president to give the other side hell, I don't know what will.

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