It’s only January. But the 2012 presidential campaign may have already had its most enchanting moment - and surely its hippest. Barack Obama sang a snatch of Al Green’s “Let’s Stay Together’’ at a fund-raiser at Harlem’s Apollo Theater 10 days ago. It was cool and it was funny. The singing was extremely good, too. The audience went wild. It was one of those little media moments in which popular culture and politics embrace.