While in Iowa on Tuesday, Hillary Clinton mentioned a policy reform that could affect the results of presidential races: Allowing ex-felons to vote. Clinton is not the first 2016 candidate to raise this issue, nor is it the first time that she’s done so. Sen. Rand Paul (R-Ky.) has repeatedly advocated for restoring voting rights for felons convicted of certain crimes. At several points while she was in the Senate, including shortly after she announced her 2008 candidacy, Clinton introduced the Count Every Vote Act, which would have restored those rights to anyone not currently incarcerated or not on parole or probation for a felony. We’re still early in the 2016 campaign, so it’s hard to know if that’s still the boundary that Clinton sets. As it stands, people who are convicted of felonies but are on parole can or cannot vote depending on where they live, since rules on felon voting differ by state. The Sentencing Project has a handy primer on the differences. In 12 states, those convicted of a felony cannot vote even after having repaid their debt to society — sometimes for certain periods of time, sometimes only for certain felonies. (In two states, Maine and Vermont, there are no restrictions on the voting rights of felons, even if incarcerated.) In total, some 5.8 million people are barred from voting in the United States because of their criminal past, according to the Sentencing Project’s data.