Barack Obama won the Georgia primary Tuesday night, the leading edge of a coast-to-coast struggle with Hillary Rodham Clinton for delegates in the grueling Democratic presidential campaign. John McCain challenged his remaining rivals for control of the Republican race. The results in Georgia were the first for Democrats on the biggest primary day in US history. Obama and Clinton, locked in a tight race, are hoping to become clear front-runners in what could be a long race for their party's nomination. It was Obama's second consecutive Southern triumph, and like an earlier victory in South Carolina, was built on a wave of black votes. The 87 delegates at stake in Georgia's primary were divided between the two candidates in rough proportion to the votes.