No, Republicans did not see slaves as human beings. They opposed the institution of slavery, or at least opposed the spread of it. Some saw it as morally abhorrent, others saw slaves as unfair competition for poor white laborers. They were against the spreading of slavery into the Western Territories because they wanted to make the Western Territories a Whites-only zone. No slaves, no free Africans, just Anglos.
The same type of people who had used slaves for farm labor now use illegals. They had to beg Trump to make an exemption in the deportation of the illegals they utilize because the 'family' farm is completely dependent on them, actual family farming being something of a distant past. And by paying them less than minimum wage while forcing them to buy goods only for them while charging them rent to live in trailers on the farm, making them not only dependent but putting them in debt to Mr Farmer for years, it's basically modern slavery, but with a little bit of pocket change to make it legal.
While it is legal to pay illegals substandard wages, those working stores, factories, construction... are not exempted from having to get paid a minimum wage and above.
Illegals are not stuffing the ballot box because that is... wait for it... Illegal.