Whenever his presidency ends, Barack Obama's legacy will be historic: posterity will know him as the first black president of a nation scarred at birth by a deep racial fault line.

But for Obama partisans, the hope of re-election lies in the idea that only with a full eight White House years, will he be remembered for the change he wrought, not just for who he was.

Though it was key to euphoria that greeted his election in 2008, Obama's race has rarely been a dominant political theme since.

Quickly, the same political dynamics faced by many of his predecessors: divided, vicious, partisan politics threatened to swamp the 44th US president.

All presidents crave the validation of a second term, but for Obama, that desire may be even more keen, as his Republican foe Mitt Romney vows to quickly reverse much of his legacy.

Obama's whole political project, the idea that America is not as divided as it seems, that a grass roots movement can change a nation from the bottom up, and that hope has tangible political power, is on the line.

"Our destiny is not written for us; it's written by us," Obama told a crowd in New Hampshire on Saturday, seeking to revive the sense of possibility that powered his first election win, but has since dissolved.

"We look forward to that distant horizon, to that new frontier. We imagine a better America and then we work hard to make it happen."

Should Obama win on November 6, much of his second term will be devoted to cementing the legacy of his first.

He will enshrine his health care reform -- the most sweeping social legislation for 50 years, which Romney promises to end on his first day in the Oval Office -- deep into American life.

Obama may get several more chances to reshape the Supreme Court for a generation, after adding two women, including the first Hispanic justice in his first term.

And the president, 51, will solidify reforms on gay rights, women's rights, student loans and financial reform and may tackle global warming and immigration reform.

Obama, now a graying, sometimes terse and wizened figure is a changed man from the beaming young dreamer who bounced onstage in early 2007, on a bitterly chill day in Illinois, and announced for president.

Rocketing from political obscurity, Obama, only a senator for two years, promised to use "the power of hope" to transform a nation -- a message he belted out to massive 2008 crowds, often moving his audience to tears.

He invoked a politics where people could "disagree without being disagreeable."

But the hope and optimism of his win over Republican John McCain barely survived the first contact with polarized Washington politics.

Obama the president emerged as an elusive figure, of many contradictions.

A Nobel Peace laureate who got US troops out of Iraq, Obama ruthlessly applies lethal force, in a drone war and the mission to kill Osama bin Laden.

Candidate Obama chided political leaders who feuded over "small things" yet lambasts his 2012 foe for a flip flopping condition he calls "Romnesia."

Obama inspired a generation into politics for the first time: but once president, appeared to disdain the grubby business of getting things done in Washington.

He was devoted to the grand gesture on the world stage -- for example his speech to the Muslim world in Cairo in 2009 -- but despite some success, his presidency hardly transformed America's place in the world.

And the hope that exploded in 2008, soon fizzled.

Four years on, Obama is locked in a grim grind to the finish with the joy of four years ago but a memory.

Some things have not changed. Obama retains the burning self confidence, -- foes call it arrogance -- and a fierce will to win.

In its way, victory on November 6 would have its own historic sweetness, should Obama defy economic blight that made other presidents one termers.

Busting convention is written in Obama's political DNA -- not for him a political apprenticeship in the Senate: he left to slay the mighty Hillary Clinton machine in the 2008 Democratic primary.

Matching dazzling oratory with a formidable grass roots network, Obama, along with cerebral aides like David Plouffe, re-invented how US elections are won in 2008.

His massive operation will redefine re-election races if he wins next week.

Obama, despite the claims of conservative conspiracy theorists, was born in Hawaii in 1961 to a black Kenyan father and white mother from Kansas. His father abandoned the family when "Barry" Obama was just two.

His mother Ann, an anthropologist who died in 1995, took her son with his new stepfather to Indonesia and he returned to live with his grandparents in Hawaii in his restless teens.

After attending an elite Hawaii academy and two colleges including Columbia University in New York, Obama went to the elite Harvard Law School and was the first African-American to edit the Harvard Law Review.

Married in 1992 to Michelle, a fellow lawyer, Obama rose through bare-knuckle world of Illinois politics then announced himself to the world at the 2004 Democratic convention.