The idea of a photo ID requirement for voters sounds simple and does well in polling, but the fine print of Minnesota's proposed constitutional amendment suggests that the impact on the state's election system could be complex and significant.

Questions lurk behind the snappy title:

What exactly is a "valid government-issued ID"?

How will a new system of "provisional voting" work?

What is the impact of "substantially equivalent identity and eligibility" standards on the state's popular system of Election Day registration?

None of the questions is unsolvable, and supporters say any changes will be for the good. But the complexity behind the "photo ID" catchphrase creates plenty of room to vigorously debate what the change will mean.

"The appeal is simple, but the implementation is not," said Doug Chapin, an elections expert at the University of Minnesota's Humphrey School of Public Affairs.

"It's more than just, we're showing people an ID," said Joe Mansky, a veteran of the state's election system. As Ramsey County elections manager, Mansky refers daily to a thick binder of election law, rules and court rulings built up since statehood. "It's serious business. What they're proposing is a substantial change to the way we're conducting elections."

Sen. Scott Newman, R-Hutchinson, a lawyer and photo ID sponsor, said the text of the amendment backs up the central idea of having voters prove their identity and residence by showing a picture ID. He said the system can and will adjust, assistance and free IDs will be provided, and voters ultimately will have more confidence in the system's integrity.

"The gravamen of the amendment is photo identification for voting," he said, using the legal term for the essence of the issue. "There are certain procedural effects that are going to occur when this amendment is adopted. ... But the purpose, the basic purpose, is photo ID."

A Star Tribune Minnesota Poll in May 2011, when the Republican legislative majority was debating the issue, showed 80 percent support for "requiring Minnesota voters to show a photo ID to vote." Support among Democrats who were polled was nearly two-thirds.

That may help explain why, after Gov. Mark Dayton vetoed the election changes as a 62-page bill sent to him in 2011, the concept re-emerged this year as a one-page proposed constitutional amendment, which skirts a Dayton veto and goes directly to voters.

But polls do not generally ask respondents about details.

More than photo ID

Minnesota does not now have "provisional voting," in which a voter casts a ballot on Election Day but must return at some point to provide documentation in order to get the ballot counted. The amendment requires provisional voting for those without an approved ID, a concept that Newman said was blessed by the U.S. Supreme Court in its ruling on Indiana's ID law.

The shorthand "photo ID" means different things to different people, but the amendment will require a "valid government-issued ID." Mansky wonders about thousands of private college students in St. Paul, and even Supreme Court justices who have inquired if their various IDs would be acceptable. Mansky, who testified on the bill on behalf of the Minnesota County Auditors Election Committee, wonders what an election judge is supposed to do if a voter's ID picture does not appear to match the real person.

The amendment says all voters, "including those not voting in person, must be subject to substantially equivalent identity and eligibility verification" before voting. Newman sees "substantially equivalent" as allowing for absentee and military voting and same-day registration to continue. "If you come in for same-day registration and show your photo ID, you're going to vote, just like you always have," Newman said.

Opponents of the amendment worry that poll workers could not possibly apply "substantially equivalent" standards to voters who register on Election Day, because there would not be the time or resources to instantly run the checks that normal registrants are subjected to. They worry that some or all of the Election Day registrants -- 542,257 in the last presidential election -- would be thrown into the "provisional" pool, causing havoc with vote-counting and verification.

Both sides do agree that a photo ID requirement will end the process of "vouching," in which registered voters can swear as to the residence of a voter who lacks identification. Supporters of the amendment say vouching is an invitation to fraud. Mansky says there has been no such evidence, and the effect might be to permanently disenfranchise a small but significant subset of the urban electorate that in a tight race could tip a contest.