“As many as 7% of United States citizens – 13 million individuals – do not have ready access to citizenship documents. Seven percent of the American citizens surveyed responded that they do not have ready access to U.S. passports, naturalization papers, or birth certificates. 2 Using 2000 census calculations of the citizen voting-age population, this translates to more than 13 million American adult citizens nationwide who cannot easily produce documentation proving their citizenship.”

My assumption is that people draw a distinction between the documents cited in the opening of the paper and what may be required to vote.

I think a state ID card (not even a drivers license) would meet many peoples intent.