Bill Clinton will have an up-front role in this summer's Democratic national convention, where he will make a forceful case for Barack Obama's re-election and his economic vision for the country, several Obama campaign and party officials have said. The Obama campaign will be keen to take advantage of the former president's immense popularity and remind voters that a Democrat was in the White House the last time the American economy was thriving. Clinton's prominent role at the convention will also allow Democrats to embrace party unity in a way that is almost impossible for the Republican challenger, Mitt Romney....