Four years ago, voters in Ohio and a few other swing states opened their mailboxes to discover a documentary they’d never ordered. “Dreams From My Real Father” posited that the president of the United States was not the son of Barack Obama Sr., but of Frank Marshall Davis, a Communist activist and poet who moved to Hawaii late in life. An Obama soundalike narrated the film, describing how Davis’s radicalism — mentioned occasionally in the real “Dreams From My Father,” referred to as “Frank” — influenced the man then seeking a second presidential term.