HARRISBURG (AP) - Voters in Pennsylvania rarely elect black and female candidates. But they'll have to choose one or the other in the April 22nd Democratic presidential primary between Senators Barack Obama and Hillary Rodham Clinton.

Political operatives and scholars don't have a consensus explanation for the political glass ceiling in a state that currently has only one black and one woman in its 21-member congressional delegation and has never had a black or female governor.

Some chalk it up to the parties' failure to recruit more women and blacks, and a tendency to favor incumbents over untested upstarts. Some theories hold that juggling young families and political careers deters women from seeking full-time office. And some believe the concentration of blacks in urban areas works against black candidates for statewide office who must seek votes in predominantly white rural counties.