Calling someone a racist is an
ad Hominem attack. It is an admission of defeat.
The moment it happens (and it happens a LOT) I reply "you're a tired old race-baiter who just lost the argument".
By shading Obama with the racial umbrella, the left is saying he ain't good enough to stand on his own merits. They are slighting him and all darker skinned Americans as not being good enough without the racial umbrella. They are being dispicable racisist in the mold of the worse slave owners and uncle Toms.
They are saying blacks are not as good as whites. They are pathetic.