If you ever saw that PBS documentary on the history behind D.W. Griffith's Birth of A Nation, including the fight of a Boston based Black man who fought (unsuccessfully) to prevent the movie from being screened in Boston (a documentary I highly recommend, if you can find it), you know that Obama's mixed heritage makes him more of a target for venom in their eyes.
According to the novel the movie was based on The Clansman, white supremacists have particular ire for 'mixed' Black men who they regard as every bit as cunning as a White man but have the beastly tendencies of a Black man (according to white supremacist logic).
This is why white supremacists and their cohort often regarded Obama as untrustworthy and dishonest (e.g. the hidden Muslim identity) and were constantly of the belief that he was going to take their guns away (no matter how often he said that he would not) and then redistribute all the wealth and give it to 'the Blacks' (we know that was never going to happen).