Barack Obama rose to power as the country's first African American president with message of hope and boundless optimism for the future.

"If there is anyone out there who still doubts that America is a place where all things are possible...who still questions the power of our democracy, tonight is your answer," he told crowds in Chicago in 2008 after winning the election.

In all the years since he never wavered from his mission to help foster in America what he once called the "renewal of morality".

Unlike Richard Nixon, Bill Clinton and so many others before him, this is a president unblemished by scandal.

The president acted presidential even behind closed doors: even his closest aides fail to recall moments when Mr Obama gave way to roiling anger. Emotions have rarely muddled the academic rigor of his mind.

White House staffers have nicknamed him the 'Colombo president', after the famous television detective who always catches the killer with his questions.