Historian Edmund Morgan called slavery the central paradox of American history and he convincingly linked the rise of liberty and equality in America and soothing of class conflict to slavery.
本句不存在省略is的问题。 本句中slavery后面的名词是作宾补的名词.本句是复合宾语句型即call sth sth 名词作宾补。 宾语从句有时可以用复合宾语代替。如I think it important to study。=I think it is important to study englsh。但这不是省略is的问题而是两个不同的句型。 另外本句call后面一般不跟宾语从句，所以本句slavery 后面不能加is/was。