Hillary Clinton is three times as popular as her most prominent challenger Joe Biden for 2016, should she choose to run. For that reason, her spokesman says she is prepared to step down from her post as Sec. of State "within days" of Obama's second inauguration.
As of right now, the prominent names in the Democrat hat for 2016 are Hillary, Biden, and New York Governor Andrew Cuomo.
Sec. Clinton's spokesman Philippe Reines says she has been honored to be Secretary of State, "and has loved every minute of leading the Department and being part of the State Family."
In the short term, once Clinton steps down, the UK Mail is reporting that Senator John Kerry could be tapped as a replacement for her at the Secretary of State post.