Mr Obama told auto workers in Toledo that his bet to bail out US car makers was "paying off for America".
The convention, which kicks off on Tuesday, is to feature speeches from the president, his wife Michelle Obama and former President Bill Clinton.
Mr Obama faces Republican nominee Mitt Romney in the race for the White House.
"I stood with American workers, I stood with American manufacturing, I believed in you," Mr Obama said in Ohio on Monday, the US Labor Day holiday.

'Good old days' Campaigning in Detroit, Michigan, a major hub for car manufacturing, Vice-President Joe Biden stuck to the same message, saying: "America is better off today than they left us when they left."
He roused the crowd with the familiar slogan: "Osama bin Laden is dead and General Motors is alive."
But in North Carolina, a state that both parties are vying to win in November's general election, Republican vice-presidential candidate Paul Ryan was on the campaign trail, offsetting the Democratic message.
"Simply put, the Jimmy Carter days look like the good old days compared to where we are now," he told supporters, referring to the Democratic president ousted from office after just term in 1980 by Ronald Reagan.
While most Americans spent Monday marking Labor Day, Mr Romney issued a statement from his lakeside home in the state of New Hampshire, saying: "For far too many Americans, today is another day of worrying when their next paycheck will come."
Correspondents say the Democratic push to highlight what they see as Mr Obama's economic achievements comes the day after a key Obama supporter, Governor Martin O'Malley of Maryland, suggested Americans were not better off than they were four years ago.
Mr O'Malley sought to clarify his remarks on Monday, telling CNN: "We are clearly better off as a country because we're now creating jobs rather than losing them.
"But we have not recovered all that we lost in the Bush recession. That's why we need to continue to move forward," with a second Obama term, he added.
'Weather gods' The three-day Democratic National Convention follows on the heels of the Republican National Convention held in Tampa, Florida, last week.
A Gallup opinion poll released on Monday suggested the convention had given the Republicans only the slightest of boosts, with 40% saying they were now more likely to vote for Mr Romney but 38% of respondents describing themselves as less likely to.
Mr Obama maintained a lead over Mr Romney of one percentage point - as he had done before the event.
Scheduled to speak on Tuesday is First Lady Michelle Obama, as well as Democratic city mayors including Julian Castro of San Antonio, Texas.
On Wednesday the podium will see speeches from Elizabeth Warren, who is fighting Republican incumbent Scott Brown in a high-profile race for a Massachusetts Senate seat, and former President Bill Clinton are to take to the podium.
The convention is to culminate on Thursday with speeches from the Democratic candidates - Mr Obama and his running mate Mr Biden.
As they did in 2008, the Democrats will take the event outside the convention centre for the president's prime-time speech, taking over a 74,000-seater stadium in Charlotte for the final night of speeches.
Organisers are working to ensure a full house for Mr Obama's speech. There will be over 6,000 delegates in attendance, and large groups of people associated with the event.
But organisers are concerned thunderstorms forecast to hit Charlotte during the convention could keep people away.

Mark Mardell North America editor

Come here for America in all its glory - my take on the twist and turns of the presidency, electoral races and life beyond Washington

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