"The U.S. intelligence community has assessed that imposing new unilateral sanctions now ‘would undermine the prospects for a successful comprehensive nuclear agreement with Iran.’ I share that view. It could rob us of the diplomatic high ground we worked so hard to reach, break the united international front we constructed, and in the long run, weaken pressure on Iran by opening the door for other countries to chart a different course," Clinton wrote in the January 26 letter.
She continued, "So long as Iran remains a sponsor of terrorism and a threat to global security, we will have to remain vigilant in defense of our allies and partners, including Israel. Yet I have no doubt that this is the time to give our diplomacy the space to work."
Clinton also warned that the passage of additional sanctions by Congress could project an image of internal discord.
"We should give anyone watching from Tehran no reason to doubt America's unity and resolve," she wrote.
On Sunday, Levin issued a response to Clinton's letter:
"This is a thoughtful, persuasive argument from an experienced, respected senior official. It makes clear Secretary Clinton's belief that tough sanctions helped bring Iran to the negotiating table, and that Congress and the administration are poised to act if Iran violates its commitments or fails to negotiate in good faith toward a final agreement. Her letter is another strong signal to Congress that we should not take any legislative action at this time that would damage international unity or play into the hands of hard-liners in Iran who oppose negotiations."
Obama has vehemently urged Congress against passing a bill imposing new sanctions as the international deal to curb Iran's nuclear program goes into effect.
"Unprecedented sanctions and tough diplomacy helped to bring Iran to the negotiating table, and I'm grateful to our partners in Congress who share our goal of preventing Iran from acquiring a nuclear weapon," Obama said last month. "Imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully, and I will veto any legislation enacting new sanctions during the negotiation."
Meanwhile, a bipartisan sanctions bill has lost steam in the Senate, as Senate Majority Leader Harry Reid (D-Nev.) keeps the legislation at bay.
"The sanctions bill is on ice while the diplomatic process plays out," a senior Senate Democratic aide told The Huffington Post. "The fact that cosponsors of the bill are now publicly distancing themselves from the measure shows just how hasty and ill-conceived this effort has been."
Read Clinton's letter to Levin: