10thAmendment?
At the beginning of the US Constitution, Article I Section 2 says about voting for the US House of Representatives, "The Electors [voters for US House reps] in each State shall have the Qualifications requisite for electors of the most numerous Branch of the State Legislature."
So that recognizes the right of each state to determine who can vote. It's subject to *specific* later US Constitutional amendments, i.e., you can't keep someone from voting based on race or sex. But there's nothing in there about paupers.
That's why the states can bar felons from voting. Some states do, some states don't. It's up to the individual state. Eric Holder doesn't like barring felons, but there's nothing he can do about it. The Constitutional principle of states determining qualifications for voting (subject only to *specific* Constituional clauses) is well-recognized.
The expert who posted to me had a lot more to back it up, but that's what I remember.