The intervention from Clinton, who holds a commanding lead in a 2016 Democratic primary contest she has not formally entered, represents both a new opportunity for the Obama administration to rescue its major diplomatic overture from a Congress that largely loathes it and a threat of being undermined by a potent and independent force within the Democratic Party.

That coalition, she implied in a letter released on Sunday, by Senator Carl Levin, was an accomplishment of Clinton’s tenure as Obama’s first secretary of state. Her letter subtly points to differences between her foreign policy outlook and that of the administration she served, a discrepancy that both Clinton and Obama will have to navigate delicately as the next presidential contest approaches.

In her letter, dated 26 January, Clinton stressed her early advocacy of “crippling sanctions” on Iran over its nuclear program, a nod to her bona fides as an Iran hawk, before telling her former Senate colleagues that proposed sanctions legislation would risk making Washington seem at fault if a permanent deal cannot be reached in the agreed-upon six-month timeframe.

“It could rob us of the international high ground we worked so hard to reach, break the united international front we constructed,” Clinton wrote in her letter, dated 26 January.

Clinton pointedly did not sound hopeful notes about the prospect of decades-old enemies reaching a peaceful resolution of a major grievance. The closest she came was urging that “this is the time to give our diplomacy the space to work”.

“At this moment it is of particular importance that our government’s efforts work in coordination, not at cross purposes. We should give anyone watching from Tehran no reason to doubt America’s unity and resolve,” Clinton wrote, positioning herself as an outside mediator bringing Congress and the White House in line.

Iran and the five permanent members of the UN Security Council and Germany struck an interim deal in Geneva on November 24 under which the US and its allies made a commitment to lift some of the existing sanctions and not to impose new ones in return for Iran making restrictions to its nuclear energy program.

The administration of President Barack Obama has clashed with Congress over the issue of Iran sanctions. Many members of Congress want to impose new sanctions on Tehran, but Obama has threatened to veto a sanctions bill.

SHI/SHI