COLLEGE PARK, MD - FEBRUARY 11:  Democratic presidential hopeful Sen. Barack Obama (D-IL) addresses a campaign rally in the Comcast Center at the University of Maryland February 11, 2008 in College Park, Maryland. Obama is campaigning ahead of the February 12 Democratic primaries in Virginia, Maryland and the District of Columbia, dubbed the 'Potomac Primary.' Getty Images logo Getty Images 21 months ago

COLLEGE PARK, MD - FEBRUARY 11: Democratic presidential hopeful Sen. Barack Obama (D-IL) addresses a campaign rally in the Comcast Center at the University of Maryland February 11, 2008 in College Park, Maryland. Obama is campaigning ahead of the February 12 Democratic primaries in Virginia, Maryland and the District of Columbia, dubbed the 'Potomac Primary.'