Democrats launched President Barack Obama on his uncertain bid for re-election, as they opened their national convention on Tuesday. They looked to help Mr. Obama recapture the hearts of Americans once drawn to his message of hope and change, but now weary after years of economic weakness and political squabbles.

First Lady Michelle Obama, drew a contrast between her husband and rival Mitt Romney without mentioning the Republican’s name. She talked about her husband’s more humble background as a young man who drove a rusty car and wore dress shoes that were a size too small.

She painted a portrait of a leader who knows first-hand the struggles of everyday Americans, who listens to them as President, and who pushes an agenda with their interests in mind.

“That’s the man I see in those quiet moments late at night, hunched over his desk, poring over the letters people have sent him,” she said. “I see the concern in his eyes ... and I hear the determination in his voice as he tells me, ‘You won’t believe what these folks are going through, Michelle. It’s not right. We’ve got to keep working to fix this. We’ve got so much more to do.’”

Polls show Mr. Obama and Mr. Romney locked in a tight race ahead of the November vote. Mr. Romney is seen as the better candidate for improving the economy, the biggest issue in the race. But Mr. Obama is seen as more likable and better able to connect with the concerns of middle-class Americans.

Tuesday’s other major speaker was San Antonio Mayor Julian Castro, who delivered the high-profile keynote address.

Mr. Castro said Mr. Obama “knows better than anyone there’s more hard work to do.” He said the United States is making progress “despite incredible odds and united Republican opposition.”

Many speakers denounced Mr. Romney. Senate Majority Leader Harry Reid repeated attacks about Mr. Romney’s refusal to release years of past federal income tax returns.

“Never in modern American history has a presidential candidate tried so hard to hide himself from the people he hopes to serve,” he said.

Other convention highlights include Wednesday’s roll call votes formally nominating Mr. Obama and Vice-President Joe Biden, and a speech by Bill Clinton, the popular former president. The climax will be Mr. Obama’s acceptance speech at a 74,000-seat football stadium on Thursday.

That speech will seek to recapture the grandeur of Mr. Obama’s acceptance address in a Colorado stadium four years ago. At the time, the United States was in the midst of a devastating financial crisis while unpopular wars were dragging on in Iraq and Afghanistan. Mr. Obama young, magnetic and eloquent captured the imagination of many Americans as the first black nominee of a major party. He promised a fresh start after eight years of George W. Bush’s presidency and new hope for the economy.

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