This chart by Sean McElwee of Demos shows why felon disenfranchisement matters. In four key Republican victories in 2014 — the Senate races in Alaska, Georgia, and North Carolina, and the governor's race in Florida — the number of votes that the Republican candidate won by was smaller than the number of citizens who couldn't vote:
Most states prohibit people from voting if they're serving sentences for a felony (whether they're physically in prison, or out on probation or parole). And over a dozen states restrict voting rights for ex-felons even after they've finished their sentences — sometimes even for life.
Those disenfranchised citizens are disproportionately African American. In Florida, for example, nearly a quarter of the state's African American voters were disenfranchised due to their criminal records as of 2012. That's because Florida is one of a few states that ban ex-felons from voting for life. In Georgia and North Carolina, where the law restores a citizen's voting rights after he's finished his sentence, many fewer black voters are disenfranchised:
There's no guarantee that all these people would vote if they could. After all, only 36.6 percent of eligible voters (according to an estimate from Michael McDonald of the United States Elections Project) showed up to vote this year. But it's also possible that, in communities where many people have done prison time and lost their voting rights — say, African-American communities in Florida, where ex-felons are disenfranchised for life — even eligible voters are less likely to turn out to vote. After all, if peer pressure from Facebook friends who voted can make people more likely to vote themselves, people with more friends who can't vote don't feel that peer pressure.
Even if these citizens would have swung all four races to Democrats if they could have voted, they wouldn't have kept the Senate in Democratic hands. But maybe they would have affected the outcomes of a few state legislature races — which now have historic Republican majorities. After all, it's the states that have set the laws disenfranchising felons and ex-felons — and absent a change to federal law, they're the only ones who can reenfranchise them.