In a letter accompanying his veto (pdf) Gov Nixon said,
This new mandate would disproportionately impact senior citizens and persons with disabilities, among others, who are qualified to vote and have been lawfully voting since becoming elible to do so, but are less likely to have a driver's license or government-issued photo ID. Disenfranchising certain classes of persons is not acceptable.The GOP has launched a nationwide campaign starting in the Bush years to require all voters to show state issued photo IDs before being able to vote. Under current law any evidence that a voter resides at the registration address, including the government issued voter card mailed to each voter prior to an election, a utility bill or bank statement is sufficient. The fact is that many poor voters, the elderly and disabled tend to vote for Democrats and this is intended to stop them from voting.
The Ohio voter ID law even goes after students attending Ohio's Universities by requiring they show a photo ID issued by the State of Ohio and an ID issued by state colleges does not count as presumably those are all fraudulent. It's outrageous and exists for the sole purpose of preventing a Democrat from winning Ohio.