A controversial constitutional amendment that would require voters to show a photo identification at the polls has cleared one of the last hurdles on the way to the fall ballot.

An all-Republican conference committee hammered out a compromise version of the House and Senate voter identification proposals and passed it unanimously Monday night, over the vocal protests of the Minnesota Secretary of State's office, which described the compromise as "worse" than either of the original plans.

Proponents were more pleased with the committee's work.

"The state has a legitimate interest in guarding the integrity of the voting system, and the perception of integrity within the voting system, while at the same time making sure to craft this legislation in such a way that it is not burdensome to those who choose to participate and vote," said Sen. Dave Thompson, R-Lakeville. "I believe that we've worked something out that complies as best we can."

The compromise plan now returns to the House and Senate for a final vote. Both chambers had earlier approved slightly different versions of the amendment on straight party-line votes. Gov. Mark Dayton can't veto a proposed amendment, as he did with a photo identification law passed by legislators last year.

Both the House and Senate versions of the amendment had three main elements: A requirement that all voters at the polls show some form of photo identification; a requirement that all voters meet "substantially equivalent" eligibility standards, and the creation of a new system of provisional ballots for those without proper ID or eligibility verification. The amendment also would commit the state to providing free IDs if requested.

All sides admit that the amendment language lacks detail. If voters approve the amendment, the 2013 Legislature would have to determine its effect on absentee and mail-in ballots, the popular system of Election Day registration, and provisional ballots.

On Monday, supporters and opponents of the bill lined up one more time to make their cases for changes to the proposed amendment. The League of Women voters petitioned for exceptions for those who have trouble getting photo identification -- people like the homeless or nursing home residents.

Beth Fraser of the Secretary of State's office was particularly unhappy to see the return of language that would require all voters to be subjected to "substantially equivalent identity verification," which she said could endanger absentee voting.

"That's 195,000 voters who vote absentee by mail in a presidential election year, 11,500 military and overseas voters and 45,000 who are registered in mail ballot precincts," she said.

Opponents worried that the conference committee language still leaves too many open questions. For instance, does the requirement for "government-issued identification" to vote mean that students at public colleges could use their student IDs to vote, while private college students could not? State Rep. Steve Simon, DFL-St. Louis Park, asked the conferees whether he could use his state-issued House ID to vote.

Dan McGrath, of the pro-photo-ID group Minnesota Majority, said it's a simple matter of "fairness and integrity" to expect everyone who votes to prove that they're eligible to vote. "I really, really don't see any problem with it," he said.

One key difference between the House and Senate versions was an amendment by Sen. John Howe, R-Red Wing, which was intended to allow for "equivalent" identification technology and not limit the ID to a photograph. House sponsor Rep. Mary Kiffmeyer, R-Big Lake, opposed that change.

Passing photo ID has been a key goal of the Republican majority in 2012. So far, the amendment has attracted no DFL support. Every member of the conference committee is Republican.

Photo ID would be the second proposed amendment to the state Constitution on the November ballot, joining one that would ban same-sex marriage.

Staff writer Jennifer Brooks contributed to this report. Jim Ragsdale • 651-925-5042#