Competition to become US secretary of state is between former presidential candidate and ambassador to UN, Susan Rice

Hillary Clinton's departure from the state department opens up a vacancy for the US cabinet's highest ranking post, and the competition has been quiet but intense.

The two frontrunners are Senator John Kerry and the ambassador to the UN, Susan Rice, with the lead apparently swapping hands more than once if Washington conventional wisdom is to be believed. Rice, a veteran diplomat and foreign policy scholar, was widely seen as the favourite for much of the year, but was seen to stumble in the final stretch over the attack in September on the US consulate in Benghazi in which the ambassador was among those killed.

She denied the attack had been premeditated, a position later disowned by US intelligence, which has since portrayed it as a terrorist assault. Rice became the target of cover-up allegations from the Romney camp, who claimed the government had tried to hide the true nature of the incident to disguise a security lapse. Clinton and Kerry came to her defence and the White House insisted she had simply been expressing the best-known facts at the time, but the incident would be likely to haunt her at Senate confirmation hearings if she got the job.

Kerry comes with his own pros and cons. As chairman of the Senate foreign relations committee and former presidential candidate, he has good credentials of his own, and his role as Mitt Romney's stand-in at Obama's debate rehearsals gave him extensive face-time with the president during the last weeks of the campaign.

Nevertheless, the wafer-thin Democratic Senate majority may mean Obama cannot move him. The requirement for Kerry to resign his Massachusetts Senate seat to take a cabinet post would trigger a special election to replace him, opening up a chance for the Republican Scott Brown to get into the Senate having been defeated on Tuesday by the Democrat Elizabeth Warren.

Other options include the national security adviser, Thomas Donilon, and William Burns, a deputy secretary of state and a former ambassador to Moscow.