Barack Obama has launched his re-election campaign in Columbus, Ohio, a major swing state that could be key to his hopes of winning November's election.
The US president said on Saturday that Democrats would win the election the "old-fashioned way" and that his team would campaign "door by door, block by block, neighbourhood by neighbourhood".
Obama will likely be challenged by Mitt Romney, the former governor of Massachusetts who is on track to gain the 1,144 delegates needed to secure the Republican nomination.
The president used his first political rally to attack Romney for learning the "wrong lessons" as a CEO and promised to move the US economy forward if he wins a second term.
Ohio and Virginia, where Obama was scheduled to campaign next, could be pivotal states in the November 6 election.
With his wife Michelle at his side, Obama said: "He has run a large financial firm and he has run a state, but I think he has drawn the wrong lessons from those experiences."
"He sincerely believes that if CEOs and wealthy investors like him make money the rest of us will automatically prosper as well," Obama, dressed in a button-down shirt without a tie or a jacket, told the crowd estimated to be more than 18000.
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He said Romney and his fellow Republicans would take the country back to the policies that led to the recession.
"We were there, we remember, and we are not going back - we are moving this country forward," Obama said.
A video highlighting Obama's political life was shown to jazz up the crowd. It included a clip of 2008 Republican vice presidential nominee Sarah Palin, followed by a shot of Obama brushing something off of his shoulder. The crowd roared.
"Forward" is the Obama campaign's latest slogan, and people in the crowd held signs with that word above their heads.
Romney cites his experience as a business executive as a strength and accuses Obama of not doing enough to turn around the economy.
Obama formally launched his Chicago-based re-election effort last year, but his official political events have been confined to fundraisers since then.
Earlier in his weekly address, Obama called for renewed focus on "nation-building here at home" after a decade of US-led war in Afghanistan.
Obama recapped his trip earlier to the central Asian nation, where he signed a strategic partnership agreement with his Afghan counterpart, Hamid Karzai, and marked the anniversary of the killing of Osama bin Laden.
"The tide of war has turned in Afghanistan. We have broken the Taliban's momentum. We've built strong Afghan security forces. We have devastated al-Qaeda's leadership," Obama said.
"After more than a decade of war, it is time to focus on nation building here at home," he said, highlighting his vision of an economy that offers a "fair shot" to everyone - especially US troops heading home from Afghanistan.
"What kind of country will they come back to? Will it be a country where a shrinking number of Americans do really well while a growing number barely get by?" Obama asked.