During the 2000 Republican presidential primary, Senator John McCain was the target of a whisper campaign implying that he had fathered a black child out of wedlock. (McCain's adopted daughter is a naturally dark-skinned child from Bangladesh). Voters in South Carolina were reportedly asked, "Would you be more likely or less likely to vote for John McCain if you knew that he fathered an illegitimate black child?".
Emphasis on the word reportedly...
That story is told so often by Democrats, that everybody now believes it is true.
It appears the so-called “black child” smear of McCain was an invention of the RATS to attack Bush and support their then-darling, McCain.