It was the democrate slaveholders that considered the black slaves less than human. Not the Republicans.
Thus the HUGE guilt they carry and need to “make it right”.
And they wanted them counted as whole people. Does Matthews think the Blacks were allowed to vote? The only reason to count people was to apportion Congressional representation. Don’t think the plantation owners polled their slaves to see how they’d like them to vote. The Abolishionists would just as soon have considered them non-human for purposes of apportionment.