The number of black and Hispanic registered voters has fallen sharply since 2008, posing a serious challenge to the Obama campaign in an election that could turn on the participation of minority voters. Voter rolls typically shrink in non-presidential election years, but this is the first time in nearly four decades that the number of registered Hispanics has dropped significantly. That figure fell 5 percent across the country, to about 11 million, according to the Census Bureau. But in some politically important swing states, the decline among Hispanics, who are considered critical in the 2012 presidential contest, is much higher:...