Four years ago in Prague, President Obama announced his desire to reduce the role and number of nuclear weapons in U.S. security policy and set an ultimate goal of a world free of nuclear arms. He returned to the Czech capital one year later to sign the New Strategic Arms Reduction Treaty (New START). The president has said he wants to do more: cut nuclear weapons further and ratify the Comprehensive Nuclear Test Ban Treaty (CTBT). Those are worthwhile goals, but achieving them will require overcoming significant challenges.

The New START Treaty limits the United States and Russia each to no more than 1,550 deployed strategic warheads on 700 deployed strategic missiles and bombers. That is a good step, but do those weapons levels make sense twenty years after the end of the Cold War? New START, moreover, covers only a part of the total nuclear arsenals of the superpowers; non-deployed (reserve) strategic warheads and non-strategic (tactical) weapons remain free of any constraint.

The superpowers each have total stockpiles numbering 4,500–5,000 nuclear weapons, more than fifteen times larger than the next nuclear weapons state. Washington and Moscow could easily reduce their arsenals by half and retain robust deterrents—and they would clearly remain top dogs in the nuclear-arms world.

In his January state of the union message, Mr. Obama stated his intention to “engage Russia to seek further reductions in our nuclear arsenals.” Press reports in early February suggested the administration was nearing a decision on reductions to no more than 1,000–1,100 deployed strategic warheads and a total of 2,500–3,500 total nuclear weapons. The administration could pursue this in different ways.

One option would seek to negotiate a U.S.-Russian treaty covering all nuclear weapons. It might limit each side to 2,500 total weapons, with a sublimit of 1,000 deployed strategic warheads. That would reduce the New START level by 35 percent and, more significantly, for the first time cap reserve strategic and tactical nuclear weapons.

Negotiating such an agreement would get into new territory; for example, the sides would need to develop agreed definitions, counting rules and verification measures to apply to the classes of warheads not previously limited. None of that would be easy and would take considerably longer than the eleven months it took to finish New START.

The Obama administration might conclude that it lacks time to finish such a treaty before the end of his second term. It thus might consider a fast deal to reduce New START’s limits. That could be as simple as just negotiating new numbers, for example, a limit of 1,000 deployed strategic warheads in place of 1,550. New START’s definitions, counting rules and verification measures would apply equally well to the new numerical limits. As for reserve strategic and tactical weapons, Washington could seek to engage Moscow in a process beginning with transparency and confidence-building measures and ultimately leading to a negotiation of legally binding limits. However, getting to that negotiation, and then concluding it, would take far longer than agreeing to change the New START limits.

Pursuing either approach would encounter challenges. The first and most critical: is Moscow prepared to engage? President Putin and the Russians have shown little enthusiasm recently for further nuclear arms cuts. They may choose not to play.

Perhaps not, but it is too early to close that door. The Russian government could have incentives to negotiate. For example, while the U.S. military can easily maintain its forces at New START levels, the Russian military must build new missiles to keep to the levels. Lowering New START’s limits could provide an attractive cost-saving measure for Moscow.

National Security Advisor Donilon travels to Moscow next week, and presidents Obama and Putin plan to meet in June and September. Those encounters provide opportunities to sound out the Russians’ readiness to deal.

Verification will pose a special challenge for limits on reserve and tactical nuclear arms. The U.S. intelligence community has high confidence in its ability to monitor New START’s limits. But monitoring constraints on reserve strategic and tactical weapons—which are not deployed on large strategic ballistic missiles but sit in storage bunkers—will prove a tougher task.

In the end, the intelligence community likely will not have the same degree of confidence in its ability to monitor those limits as it does with New START. That will raise questions, particularly in the Senate, though the risk posed by less certainty in monitoring limits on reserve strategic and tactical weapons should be set against the current situation, in which there are no constraints of any kind on those weapons.

A third challenge waits on Capitol Hill. Senate turnover has meant the loss of considerable muscle memory on nuclear arms-control questions. Senate Republicans, moreover, tend to be skeptical about the value of arms control. And they feel that the Obama administration has not moved as fast on nuclear modernization as it promised during the New START ratification debate. So, any new nuclear-reductions treaty would face a stiff test in a ratification vote.

That has led the administration to weigh options other than a legally binding treaty. One could be to seek a political commitment by the U.S. and Russian presidents to cut deployed strategic warheads to one thousand on no more than five hundred deployed strategic delivery vehicles. The sides could use New START’s verification measures to monitor these politically binding limits as well as the legally binding limits of 1,550 and 700.

The administration also would like to secure ratification of the 1996 CTBT. The Senate did not consent to ratification in 1999, primarily due to concerns about the reliability of the U.S. stockpile absent testing and the ability to detect cheating. Developments over the past ten years in the stockpile-stewardship program and advances in monitoring, such as improved seismic techniques, have largely allayed those two worries.

Bear in mind also how hard Nevada fought against storage of nuclear waste at the nuclear test site. With the population of nearby Las Vegas having tripled since 1992, the year of the last U.S. nuclear test, does anyone believe a resumption of testing would be feasible politically? Moreover, the United States carried out more nuclear tests than the rest of the world combined and learned more from individual tests. Why not freeze this American advantage?

Persuading Senate Republicans of the validity of these points nevertheless cannot be taken for granted. The administration will want to do a careful head count before making a CTBT ratification push, as a second negative vote in the Senate would be devastating for the treaty.

The president has said he wants to do more on cutting nuclear-arms levels and moving the CTBT closer to reality. Those are worthy goals that could cement his nuclear legacy and make America more secure. But major challenges stand before his agenda. President Obama has to engage personally, both with the Russians and the Senate, if he wants to overcome them.

Steven Pifer directs the Brookings Arms Control Initiative and is co-author of The Opportunity: Next Steps in Reducing Nuclear Arms.