Barack Obama won the Virginia primary Tuesday night and reached out for victories in nearby Maryland and Washington, D.C., determined to erode or even erase Hillary Rodham Clinton's delegate lead in the Democratic presidential race. Obama's triumph made it six straight over Clinton, the former first lady, now struggling to keep up in a race she once commanded. The Associated Press made its call based on surveys of voters as they left the polls. The interviews with voters showed blacks accounted for nearly one-third of the ballots cast in Virginia, and Obama's share approached 90 percent. He and Clinton split the white vote.