Charlotte, North Carolina (CNN) - A Democratic National Convention vital to President Barack Obama's re-election bid opened Tuesday with the more than 5,500 delegates adopting a party platform that emphasizes middle-class opportunity while differing sharply on key issues with the Republican version endorsed at last week's GOP convention.

Facing a tight race in November and Republican attacks that they have made things worse while in power, Democratic organizers want their three-day convention to emphasize the tough decisions Obama has made and the additional steps needed to bolster the middle class.