Several key states are introducing tighter voter ID requirements this year, which many see as a thinly-veiled effort to suppress voter turnout. Corey Dade explains why more than three million Americans don't own a government-issued picture ID: 

[T]he first thing to look at is to look at who actually drives. The most common form of government-issued ID are driver's licenses and so the people who are most unlikely to drive, as it is, is elderly, the poor, people who live in big cities, like African-Americans, especially young people, too, especially if they attend college. They may not have need for a car at the moment. And then people who are in rural areas. The other challenge for them is they are not near the Department of Motor Vehicles offices, etc., where you would get these IDs. 

Debbie Hines has more

The Brennan Center estimates that 18 percent of all seniors and 25 percent of African-Americans don't have picture IDs.

Estimates are that it could cost up to $35 to obtain the necessary documents when not in the possession of the voter, plus time off from work to obtain during working hours. And with many people struggling due to the economy, where many sometimes don't have enough money to buy food and put and keep a roof over their head, it's unlikely they will have an extra money to pay to vote. Although the government-issued photo ID needed to vote is free, the certified birth certificate or passport needed to obtain one, if unavailable, costs money. And there is additional cost to obtain even more documents, in those cases where a name change is involved due to marriage or divorce.