A letter by 47 U.S. Republican senators to the government of Iran has backfired spectacularly. Ostensibly, the letter was intended to warn Tehran of opposition to the nuclear deal that is being negotiated with Washington and five other governments — Britain, China, France, Germany and Russia. It has instead exposed the Senate Republicans as irresponsible amateurs in the practice of foreign policy and introduced partisan politics into the discussion of the deal, fracturing the opposition that is needed if the skeptics’ view is to prevail.

The “P5+1” negotiations over the Iranian nuclear program are heading toward a final conclusion. The talks now have a deadline for the end of this month, a date that is likely to stick having been twice moved back. The particulars of a deal remain unknown, but the final document will restrict Iranian capabilities without eliminating them completely. Iran’s ratification of the Nuclear Non-proliferation Treaty (NPT) permits the country to possess a peaceful nuclear program and Tehran has warned that it will maintain that right as an assertion of its sovereignty.

For some, that is impermissible, the terms of the NPT be damned. Israeli Prime Minister Benjamin Netanyahu sees any nuclear capacity in Iran — no matter what the size or purpose — as unacceptable and he used his speech to a joint session of Congress two weeks ago to hammer home that point. While Netanyahu sees any Iranian nuclear capability as an existential threat to his country, he provided no alternative means to cap Iran’s nuclear ambitions.

The GOP senators share Netanyahu’s fears, and yet have the same poverty of ideas when it comes to ways to otherwise deal with Tehran. Instead, they took the unprecedented step to publicly warn Tehran that any deal signed by President Barack Obama could be modified by Congress or reversed by a future U.S. president. In so doing, they have undermined their position on the deal and badly damaged their international status.

The conduct of foreign policy is the province of the U.S. president. Congress has a variety of roles to play in that process — ratifying treaties, providing advice and consent on the nomination of ambassadors, funding the foreign policy apparatus — but the day to day operations of diplomacy and international engagement, negotiations in particular, are the preserve of the executive branch. The interjection of GOP senators into ongoing negotiations in a way that is explicitly designed to undercut the official U.S. government position has been variously described as “a stunning breach of protocol,” “mutinous” and even “traitorous.” Without doubt, a similar foray by Democratic senators that undercut negotiations being conducted by a GOP president would provoke howling denunciations by these very signatories.

What should be more troubling to the signatories is the damage they have done to their cause. There is bipartisan opposition to the nuclear deal, but the way that the GOP rejectionists have chosen to act is designed to undermine Obama, and that rankles Democrats who might otherwise share Republican concerns about the deal. As a result, opposition to the negotiations looks partisan rather than an assessment of the merits of the negotiations — the outcome of which, after all, is not yet clear.

And since the negotiations are multilateral — not just between the U.S. and Iran — the GOP letter has undercut the party’s credibility with those other negotiating partners as well as “risks undermining the confidence that foreign governments in thousands of important agreements commit to,” explained Secretary of State John Kerry in testimony to the Senate Foreign Relations Committee last week.

That credibility was also damaged by the fact that the Senate cannot modify an executive agreement signed by the president and his international counterpart. In other words, the GOP senators’ understanding of their own power is wrong.

Finally, there is widespread agreement that Iran is ready to make a deal because of the pain that has been imposed by international sanctions. The threat of continued or even heightened sanctions is the most powerful inducement to a successful outcome. But sanctions must be multilateral to succeed. If there is a suspicion that the U.S. is not negotiating in good faith — as the GOP position suggests — then the international unity that makes sanctions possible will erode and with it the best hope to prevent Iran from acquiring a nuclear capability.

In the face of a strong domestic and international backlash, GOP leaders have suggested that they may have acted hastily — some senators said they signed the letter in a rush to get out of town before a snowstorm — or, in more candid moments, conceded that the letter was a mistake. The bell cannot be unrung, however, and the U.S. must again struggle to heal a self-inflicted wound. This is no way to conduct foreign policy and it augurs poorly for a party that seeks to regain the White House in 2016.

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