I came across this and wondered what others think about the subject.
As far as I can see these claims are rumor and innuendo with no positive proof.
http://www.post-gazette.com/pg/08036/854713-51.stmmagazines and newspaper articles found on the Web name five other U.S. presidents who may have had black ancestry, but never publicly acknowledged it: Andrew Jackson, Abraham Lincoln, Warren Harding, Calvin Coolidge and Dwight D. Eisenhower
http://www.ferris.edu/JIMCROW/question/nov08/The issue of race was injected into the 1920 presidential campaign as part of an effort to sink the campaign of Harding, the Republican nominee