Black clergy from churches across the country are telling their church members to stay home on election day. They believe that neither presidential candidate is worthy of their vote because Romney is a Mormon and Obama supports gay marriage.
This could be much more harmful to Obama because he has to rely on a large turnout of black voters in order to win.
In 2008, California had Proposition 8 on the ballot, which made gay marriage unconstitutional. It passed in large part due to the high black voter turnout because of the presidential election. Blacks have historically been anti-gay marriage.
Obama's flip flop on supporting gay marriage was to satisfy his leftwing base--totally disregarding the black vote (assuming he had it anyway).

