In nuclear negotiations, as in medicine, a core principle is: "First, do no harm." 

Unfortunately, the "Iran Nuclear Weapon Free Act" (S. 1881) introduced by Sens. Bob MenendezRobert (Bob) MenendezSchumer: Obama 'very amenable' to helping Senate Dems in midterms The Hill's Morning Report: Can Trump close the deal with North Korea? Senate must save itself by confirming Mike Pompeo MORE (D-N.J.) and Mark KirkMark Steven KirkThis week: Trump heads to Capitol Hill Trump attending Senate GOP lunch Tuesday High stakes as Trump heads to Hill MORE (R.-Ill.) would impose new sanctions on Iran and conditions for negotiations on its nuclear program that would derail the breakthrough nuclear deal between the United States and its P5+1 partners and Iran and further complicate the negotiations for a final agreement to prevent a nuclear-armed Iran.

The Nov. 2013 first phase P5+1/Iran agreement has, for the first time in a decade, paused Iran's most worrisome nuclear activities in exchange for limited and reversible sanctions relief. Even more importantly, the first phase deal opens the way for talks on a comprehensive, final agreement. Negotiations resume in Vienna on Feb. 18.

The president vowed in his State of the Union address that "if this Congress sends me a new sanctions bill now that threatens to derail these talks, I will veto it." A growing number of Senators and national security leaders have spoken out against S. 1881 and the number of cosponsors has stalled at 59. 

Serious policymakers understand that this is not the time for new Iran sanctions or other legislative efforts that attempt to dictate exactly how the United States and our partners -- China, France, Germany, Russia, and the United Kingdom -- negotiate verifiable limits on Iran's nuclear capacity that stop it short of building nuclear weapons and increase our ability to detect, deter, and respond to any effort to do so in the future.

Nevertheless, 42 Senate Republicans have demanded that Majority Leader Reid (D-Nev.) allow a vote on the bill. Menendez, meanwhile, spoke on the floor recently in defense of his bill, claiming it will help negotiators secure a better deal. It won't. 

Contrary to the claims  of the authors of S. 1881, their legislation would violate the terms of the P5+1/Iran first phase agreement announced in Geneva on November 24, 2013. In that agreement, the United States committed to "refrain from imposing new nuclear-related sanctions." S. 1881, if approved, would would impose further sanctions on Iran in several areas - including greater restrictions on oil imports and expanding business and financial sanctions on Iran's mining and construction sectors. 

Additionally, the bill sets other conditions that must be met for the president to waive the S. 1881's new sanctions exceed the terms of the P5+1/Iran nuclear deal that very likely cannot be met. Specifically, the bill requires Iran to adhere to limits on ballistic missile testing and the prohibition of financing for terrorist groups acting against the United States. 

The international sanctions imposed on Iran have played a role in motivating Tehran's leadership to reach the first-phase deal agreed to in Geneva on November 24. This success, however, was due to solid international support for unilateral sanctions imposed by the United States and the European Union. Moving forward on this bill threatens to erode support for existing sanctions amongst Washington's five other negotiating partners that could fracture sanctions enforcement.

Further sanctions are not necessary at this time. The existing, core sanctions regime provides more than sufficient leverage on Iran to take further concrete measures to restrain its nuclear potential and improve transparency measure necessary to guard against a secret nuclear weapons effort in the future.

If Iran violates the November 24 agreement or talks fail to produce a comprehensive agreement, Congress can quickly pass new sanctions--within days--and the administration says it would fully support further sanctions measures under such circumstances. 

The bill also sets out unrealistic demands for the comprehensive deal that Iran will not accept and contradict the broad parameters of the agreement laid out in the November 24 deal. 

According to the November 24 agreement, the comprehensive, final phase agreement will include a "mutually defined enrichment program" for Iran. However, S. 1881 seeks to impose a different outcome by allowing the suspension of sanctions only if Iran agrees to zero-enrichment and complete dismantlement of its "illicit nuclear infrastructure" include Iran's uranium enrichment facilities.

Such an outcome may have been conceivable in 2005-2006 when Iran agreed to suspend enrichment work and had less than 300 centrifuges. But today, demands that Iran permanently halt uranium enrichment are unrealistic and unattainable. A deal that bars Iran from enriching uranium for peaceful purposes would be unsustainable politically inside Iran-and such an outcome is not necessary to guard against a nuclear-armed Iran if appropriate limits and monitoring mechanisms are put in place.

Efforts by the U.S. Congress to move the goalposts for the final phase negotiations beyond the parameters already established by the P5+1, or to define what is or is not "acceptable" on the range of key issues under negotiation would undermine prospects for a workable, effective final phase agreement

If negotiations on that agreement falter, the risks of an unconstrained Iranian nuclear program and a conflict over Iran's nuclear program will grow.

Congress has a vital role to play in solving U.S. foreign policy challenges. S. 1881, however, would complicate international efforts to secure a sustainable, verifiable diplomatic agreement that significantly reduces Iran's capacity to produce nuclear bomb material and increases international inspection authority to help detect and deter any potential secret nuclear weapons effort in the future.

Kimball is executive director of the independent, non-partisan Arms Control Association based in Washington, D.C.