First lady Michelle Obama acknowledged on Tuesday that the change her husband Barack Obama championed in his White House campaign four years ago has proven difficult but urged voters to give him four more years to fix the struggling U.S. economy."He reminds me that we are playing a long game here, and that change is hard, and change is slow, and it never happens all at once,” she told the Democratic National Convention in Charlotte, North Carolina. "But eventually we get there. We always do.”The popular first lady was the highest-profile advocate for her husband in the first of three days of speeches that will conclude with Obama’s address on Thursday to accept the Democratic presidential nomination to face Mitt Romney on November 6.In a race that is too close to call nine weeks before Americans vote, Obama is vulnerable to the challenge from Republican nominee Romney due to a sluggish economy and 8.3 percent unemployment.Obama’s economic argument got a little tougher on Tuesday. New surveys showed U.S. manufacturing shrank at its sharpest clip in more than three years last month, while exports and hiring in the sector also slumped.The president is trying to use his convention to recapture the magic that carried him to victory in 2008 but he admitted to a Colorado television reporter that he would give himself a grade of "incomplete” for his first term.