CINCINNATI - US first lady Michelle Obama rallied supporters to back her husband as early voting began on Tuesday in the key electoral swing state of Ohio where the Democrats hope to take advantage of a lead in opinion polls.

"Are we going to just sit back and watch everything we worked for and fought for just slip away?" she asked a boisterous crowd of 6,800 in downtown Cincinnati.

With President Barack Obama focused on Wednesday's debate against Republican challenger Mitt Romney, the first lady led the Democrats' charge in Ohio, directing supporters to march to a local election office and cast ballots 35 days before the main voting day of November 6.

"All of our hard work, all the progress that we made, it is all on the line this November," she said. "Here in Ohio, it is already Election Day."

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