Yeah I think the main thing was gerrymandering in favour of the planter class and their slave populations rather than disenfranchisement of poor whites per se, at least by the eve of the Civil War. Some states like South Carolina had the three fifths rule for apportionment in the state legislature, which makes zero sense as it was supposed to be a clause in the federal constitution protecting slave states from free!
W.E.B. DuBois talks a bit about this and poor whites in the antebellum South in general in the first part of his excellent book Black Reconstruction.
South Carolina had electors chosen by state legislature for some reason back then.
Unsurprisingly, the ruling Slave Power in South Carolina were not big fans of democracy.
Especially for black people.
South Carolina was one of the first states to institute a racial requirement for voting, and prior to c. 1800 was one of only three states to bar free blacks from voting (the other two being Delaware and Virginia). As late as 1832, free black men in North Carolina and Tennessee could vote as long as they met the property or taxpaying requirement —which admittedly was not a large share of the black population, but it is an interesting feature of what was otherwise a very undemocratic political system in the former state.
That was almost certainly gone by the time of the war.
Yes, TN repealed black suffrage in 1834; NC followed suit in 1835. In 1861, only five states (New England less Connecticut) gave black men the right to vote unconditionally; three more (NY, MI, OH) allowed black men to vote if they met a property requirement, or only in certain elections.