That site is on the top of the Google search. The ultimate source is this
CNN article that I had to find in the wayback machine; the smears were attributed to a campaign writer called James Callendar, who according to the article spent time in jail for slandering Adams and exposed Jefferson’s affair with Sally Hemings as payback for lack of support. As for historical record, the author of the CNN article, Kerwin Swint (a “political scientist”), appears to have no citations.