Felons: Photo ID supporters cite convictions for fraudulent voting by ineligible felons, as well as ineligible voting by felons who weren't charged. Response: Most are mistakes and not attempts to affect elections; a photo ID requirement would not address this problem, but better notification and database checking would help.
Phantom voters: The addresses of some voters who register on Election Day cannot be confirmed after the election, even if normal excuses are eliminated. Response: In most cases, these are due to moves without forwarding addresses, postal errors or other issues; those without good explanations are forwarded to prosecutors.
No ID required: No ID of any kind is required of the vast majority of voters -- those who come to the polls and are pre-registered. This raises questions about possible voting impersonation, which cannot be checked without an ID requirement. Response: If such impersonation occurred, it would be reported, and it isn't. Voters swear an oath as to their identity and eligibility when they sign in and are guilty of a felony if they cheat.
Vouching: The ability to "vouch" for voters without proof of residence in the district opens the system to fraud and has triggered a number of complaints of questionable mass vouching. The photo ID requirement would end most vouching. Response: Most vouching is by people who live in the same residence as the person they are vouching for. It is also useful for nursing homes and other care facilities.
Election Day registration: Anti-fraud activists believe this system can permit ineligible people to vote too easily. Response: It is a popular and successful system that keeps Minnesota at the top of turnout lists, and there is little hard evidence of problems.