Introduced by popular First Lady Michelle Obama, the president will seek to fire up the vast army of supporters that swept him into the White House four years ago, with "Ready to Go" rallies in swing states Ohio and Virginia.
The president is an older, wiser and perhaps less exciting candidate than the exhilarating prophet of hope and change who rocketed to prominence when he launched his first presidential campaign on a chilly winter day in 2007.
He is scarred by three-and-a-half years of political combat in hyper-partisan Washington and the worst economic crisis since the 1930s, which doused the euphoria which greeted his historic 2008 election win.
Now, Obama faces a complicated reelection fight in a polarized nation haunted by high unemployment, and sluggish growth has robbed him of the argument that he has restored nationwide prosperity.
But Obama plans to make the case that he averted a second Great Depression and that his rich Republican foe Mitt Romney wants to return to the policies of tax cuts for the wealthy and low regulation that unleashed the crisis.
"I don't think we should prioritize things like more tax cuts for millionaires while cutting the kinds of investments that built a strong middle class," he said in his weekly address.
He will also sketch a vision of an economy that benefits all Americans, not just the rich, and laud his foreign and security policies, including the killing of Osama bin Laden, which he says have kept Americans safe.
Republicans have scoffed at Obama's "official" launch, accusing him of conducting months of campaigning under the guise of official events linked to his presidential duties.
Obama joked last week while appearing with popular former president Bill Clinton that every candidate said their next election was the most important in US history, but then turned serious.
"But let me tell you this one matters. This one matters. This one matters," he said.
An average of national opinion polls by the RealClearPolitics website shows Obama with a narrow three-point lead over Romney -- 47 to 44 percent -- six months before election day on November 6.
The president's approval rating generally sits in the high 40s, just below the 50 percent figure that presidents need to feel confident about reelection.
Obama seems to have an easier route to power than Romney on the US political map, with a number of credible paths to the 270 electoral votes needed to win the election in a dozen or so battleground states.
The president is seen as more likable than Romney in most polls, and enjoys edges on national security, and among crucial demographic groups like women and Hispanic voters that could decide the election.
But he is vulnerable on the key issue -- the economy, with unemployment over eight percent, amid confusing signals about the state of the recovery and sharp attacks from Romney, who is running under a slogan of "Obama Isn't Working."
On Friday, Obama suffered a blow when a second straight month of disappointing job creation was revealed in Labor Department figures showing the economy pumped out only a net 115,000 jobs in April.
"It's a terrible and very disappointing report," Romney said, seizing the opening in an interview with Fox News on Friday.
"Clearly, the American people are wondering why this recovery isn't happening faster," he said, in the knowledge that poor economic news represents his best chance to send Obama home to Chicago as a one-term president.
In 2008, Obama ran as a candidate of reform, change, and hope -- as an insurgent outsider who would shake up bitter Washington politics and transform the country.
As an incumbent, he no longer has that luxury: the economic problems he inherited and has struggled to solve are now part of his legacy, and he must convince Americans that things are getting better and he should get the credit.
He believes Americans will buy his economic vision of ensuring "everybody gets a fair shot and everybody is doing their fair share, and everybody is playing by the same set of rules."
Obama's campaign has unleashed a barrage of negative advertising, targeting Romney's weaknesses with women, and using his career as a wealthy venture capitalist to suggest he does not share the concerns of the middle class.
But Romney counters that his business career is exactly what is needed to kickstart the economy.
"The president's a nice guy, but he's never had a job in the private sector. He's never created a job. I think it helps to have had a job to create a job," Romney said Friday.
