By early September, P5+1 diplomats (from the United States, the United Kingdom, France, China, Russia and Germany) will likely resume talks aimed at resolving concerns about Iran’s nuclear program with President Hassan Rowhani’s new negotiating team. The talks represent an important opportunity to finally reach a deal that limits Iran’s most worrisome uranium enrichment activities, obtains more extensive inspections to guard against a secret weapons program and shows Iran a path toward phasing out international sanctions.

Unfortunately, in their attempt to encourage President Obama “to bring a renewed sense of urgency to the process,” a group of 76 senators, led by Foreign Relations Committee chair Robert Menendez (D-NJ) and Lindsey Graham (R-SC), has sent a letter to Obama that could undermine efforts to resolve the long-running dispute. The letter’s prescriptions may prompt the same kind of counterproductive impact in Iran predicted by Paul Pillar in The National Interest after House passage of H.R. 850, the “Nuclear Iran Prevention Act.”

The Menendez-Graham letter emphasizes toughening sanctions and making military threats more credible, just as Iran installs a president elected on a platform of renewing diplomatic engagement and putting an end to international isolation. The senators’ approach represents a serious misunderstanding of political realities in Iran and the nature of upcoming nuclear negotiations.

The June 2013 election results show that outgoing President Ahmadinejad’s handling of the economy and the international isolation that has resulted from his confrontational policies dissatisfies Iranians. Newly elected president Rowhani has signaled a willingness to accept greater transparency in Iran’s nuclear activities in exchange for acceptance of Iran’s rights to enrich uranium for peaceful purposes. This is the basic framework within which a negotiated resolution to the Iran nuclear crisis is possible.

With tough economic sanctions now in place, and a broad political consensus holding among the six powers negotiating with Iran, the international community has the leverage it needs to achieve an acceptable agreement. However, the senators’ letter argues that more sanctions and more credible military threats will persuade Tehran to make a deal.

Sanctions have certainly affected Iran’s economy and the government’s risk/benefit calculations, but they will not by themselves halt Iran’s nuclear program. It is fantasy to believe otherwise. The implementation of still tougher sanctions at the outset of renewed talks would harden Iran’s resolve and undermine the prospects for persuading its leaders to compromise.

Likewise, overt threats of military attack would undermine P5+1 solidarity and reduce the likelihood of Iran agreeing to limits on its program. In any case, military action—short of a permanent occupation—cannot prevent a nuclear-armed Iran. In fact, military and intelligence experts agree that striking Iran’s nuclear facilities would only delay Iran’s program by two to three years and trigger an Iranian decision to openly build nuclear weapons.

Advances in Iran’s enrichment capabilities and the start up of a new reactor next year make it important to reach a deal that limits Iran’s bomb-making potential as soon as possible. But the senators’ assertion that “the time for diplomacy is nearing its end” is naïve and unhelpful. Given the deep distrust on both sides of the negotiating table, negotiations will not likely produce immediate results. Furthermore, security experts assess that if Iran chooses to actually build nuclear weapons, it would take at least a year and probably two years to produce enough fissile material, manufacture the warheads and integrate them on ballistic missiles to field a credible arsenal.

The Senators’ letter also repeats an old and now unrealistic demand: the suspension of all enrichment activities. While this would have been an ideal confidence-building measure to facilitate achievement of an ultimate agreement, it is no longer feasible. Realists know that the negotiating issue at stake is not whether to allow enrichment, but rather the circumstances under which enrichment can occur. Secretary of State Hillary Clinton already testified to Congress in March 2011 that Iran had a right to enrich under certain conditions. Today, the P5+1 is insisting on sufficient transparency for nuclear activities inside Iran so that the International Atomic Energy Agency can be confident Tehran is not developing nuclear weapons and on placing sufficient limitations on enrichment that Tehran has no quick break-out options.

The senators also argue that the goal of U.S. policy should be that “we will not allow Iran to acquire a nuclear weapons capability.” This assertion ignores the fact that since 2007, the U.S. intelligence community has consistently assessed that Iran has the capability to develop nuclear weapons, but that Tehran had not made a decision to do so. It is reasonable to assume that Tehran wants to have an ability to build nuclear weapons quickly, but does not necessarily intend to leave the Nuclear Nonproliferation Treaty. Opinion polls in Iran indicate strong public support for Iran’s “right” to enrich uranium, but only a minority of Iranians favors the building of nuclear weapons.

The Supreme Leader, Ayatollah Ali Khamenei, may wish to leave more options open than any P5+1 deal would allow. But if the Iranian negotiators can strike a compromise acceptable to the six powers, Rowhani will need to convince Khamenei and other regime elements that Iran’s honor has been preserved and its national interests protected.

The Menendez-Graham letter may be well-intentioned, but its emphasis on threats and demands is misplaced. As President Obama argued in March 2009, “[the diplomatic] process will not be advanced by threats.” The president was right then, and the advice still resonates now.

The United States would be wise to seize the opportunity presented by Iran’s new president, reinvigorating diplomatic efforts to secure a verifiable agreement—on the basis of realistic and achievable goals—that ensures Iran does not develop nuclear weapons.

Greg Thielmann is a senior fellow of the Arms Control Association, and former office director for strategic, proliferation, and military affairs in the State Department’s Bureau of Intelligence and Research.