He wrote in his private journal in 1820:[201]

.mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 40px}.mw-parser-output .templatequote .templatequotecite{line-height:1.5em;text-align:left;padding-left:1.6em;margin-top:0}The discussion of this Missouri question has betrayed the secret of their souls. In the abstract they admit that slavery is an evil, they disclaim it, and cast it all upon the shoulder of Great Britain. But when probed to the quick upon it, they show at the bottom of their souls pride and vainglory in their condition of masterdom. They look down upon the simplicity of a Yankee's manners, because he has no habits of overbearing like theirs and cannot treat negroes like dogs. It is among the evils of slavery that it taints the very sources of moral principle. It establishes false estimates of virtue and vice: for what can be more false and heartless than this doctrine which makes the first and holiest rights of humanity to depend upon the color of the skin?
In 1836, partially in response to Adams's consistent presentation of citizen petitions requesting the abolition of slavery in the District of Columbia, the House of Representatives imposed a "gag rule" that immediately tabled any petitions about slavery. 