Hillary Clinton, the former secretary of state and Democratic presidential heir apparent, has urged her former Senate colleagues not to pass new sanctions on Iran, even as she called the Islamic Republic a “threat to global security”.

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.

Clinton’s involvement in the Iran debate subtly positions the Democratic frontrunner as an Iran hawk who is less hopeful of the diplomatic bargain ending US grievances with Tehran than she is cautious about Washington fracturing a diplomatic coalition needed to enforce punitive measures on Iran.

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 programme, 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, and in the long run, weaken the pressure on Iran by opening the door for other countries to chart a different course,” Clinton wrote.

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”.

That contrasts sharply with John Kerry, her successor as secretary of state and the driver of the Iran overture, who said in Germany this week that the world is “at a crossroads with respect to the relationship with Iran”. Instead, Clinton wrote that she has no “illusions” about Iran.

A nuclear accord, Clinton wrote, “will not suddenly remove every other concern we have with the Iranian government’s behaviour, whether in Syria, in the wider region, or towards its own people. So long as Iran remains a sponsor of terrorism and a threat to global security, we will have to remain vigilant in defence of our allies and partners, including Israel,” whose prime minister, Binyamin Netanyahu, has called the Iran diplomacy a “historic mistake”.

She continued: “Yet I have no doubt that this is the time to give our diplomacy the space to work. If it does not, there will be time to put into place additional sanctions in the future, with greater international support necessary to ensure enforcement, and to explore every other option on the table,” a diplomatic euphemism for war.

Clinton also offered special praise for Congress’s “leadership”, another departure in tone from Obama, whose White House has called sanctions advocates warmongers and who threatened in his State of the Union address on Tuesday to veto new sanctions legislation.

“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.

Should negotiations fail, she wrote, “the legislative and executive branches will move with speed and unity, backed by America’s allies, to institute even tougher sanctions”, a suggestion that punitive measures on Iran will remain an option should Clinton be elected president.

Levin, the chairman of the Senate armed services committee who said he solicited Clinton’s views, called her letter “a thoughtful, persuasive argument from an experienced, respected senior official”.

He said: “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 hardliners in Iran who oppose negotiations."

A Washington Post-ABC News poll released on 30 January found Clinton with the support of 73% of Democratic primary voters, a six-to-one margin over her closest presumed rival for the nomination, Vice-President Joe Biden. It is one of the most commanding early leads in recent presidential polling memory, and one that speaks to an enormous well of support for Clinton, whom Obama narrowly defeated for the 2008 Democratic nomination.