Liberals are really stupid and uneducated. The entire 3/5 of a person written into the constitution was a compromise which increased the representation from Demonrat (slave) states out of proportion compared to the number of actual citizens.
By counting illegals, the old Demonrats are up to the same nonsense, just using different terms. Counting illegals distorts the count and dilutes the representation and votes of true citizens.
And back then the slaves couldn't vote. So that increased the representative power per 1,000 votes. Much like today where high immigration states have a higher representative power per 1,000 votes (assuming immigrants aren't allowed to vote in federal elections).
It also, depending on how you view it, increased (or decreased) taxation.